Hi! I'm not a nutrition expert, but I'll have a go...
Nuggets vs muesli:
Nuggets: The benefit of nuggets is that every mouthful is exactly the same, so it's impossible for a rat to selectively eat only their favourite bits. The drawback of nuggets is that every mouthful is exactly the same, so for an animal that has evolved to be an adaptable, omnivorous scavenger, it's probably pretty dull. Rats and humans are not too dissimilar in our tastes (hence why they do so well hoovering up the scraps in our cities) and think how dull it would be if we had to eat a monocomponent diet! Additionally, some nuggets use pretty poor quality ingredients. That's a problem with many commercial pet foods in general, which brings me to...
Mueslis: Many commercial mueslis are bulked out with cheap but not particularly nutritious filler (many nuggets do the same, but in a nugget the filler is mixed in with everything else so can't be avoided). Grass/alfalfa pellets are a particularly notorious example of this: they don't taste very good (at least, I tried one once and it was vile
) and rats can't get much nutrition out of them either, since they've not evolved to be particularly good at digesting fibrous plant matter, yet almost all commercial rat mueslis include them. Most rats quite sensibly refuse to eat them, so some of the selective feeding with commercial mueslis is simply them objecting to unpalatable and non-nutritious filler.
However, selective feeding will be a problem even with top quality, 100% nutritious muesli mixes if the rats are fed too much
. If they're provided with an effectively unlimited supply of food, then of course they'll eat the tastiest bits only and leave the not so tasty bits, since more food will be added before they get hungry again. However, if their food supply is limited to only the quantity they need, then hunger will eventually move them to be a little less demanding; at that point you'll find out which bits they're not eating initially because they're being picky (for my rescue girls, it's their veg, like stereotypical children
), and which bits they're not eating because it's not actually food for them (like alfalfa pellets). A good quality muesli fed in the right quantities should leave little or no waste. (The only thing my rats leave is the dried garlic flakes that I keep putting in their mix anyway because I don't know what else to do with the little packet I got ages ago
but it's fair enough that they refuse the funny-tasting dried allium, really. Everything else disappears.)
The one thing that can't be prevented with muesli mixes is the possibility that different rats in the same group will have different tastes and therefore end up eating a different balance of ingredients. For example, the rescue girls I started out with used to eat their dried veg with the utmost reluctance, but after I added some breeder girls to the group, suddenly the dried veg started disappearing much faster than it used to. I suspect therefore that most of the dried veg is going into the breeder girls, but nobody seems the worse for it, so I'll not switch to a nugget just because of that. I do however keep some Science Selective Rat nuggets in stock because they're quite popular with my girls (they seem quite tasty, though I don't know if they'd still be so tasty if they were the only food for months on end), so I incorporate it into my mixes from time to time. They're also great for hiding around the free range area – easier for me to retrieve uneaten nuggets afterwards than scatterings of uneaten muesli.
Dandelion leaves and flowers:
I don't know how often is too often for that sort of thing. Dandelion is allegedly a diuretic so I suspect there is such a thing as too much of it, but I don't know how much one leaf is going to do. Mine get the odd handful of dandelion from time to time, but not that often. I suspect so long as you're not going crazy it's fine?
Raisins and treats in general:
Dried fruit is quite high in sugar, so I wouldn't give too much. Mine get probably a few pieces of dried fruit each per week; I don't know if that's a lot or a little, but nobody's keeled over yet?
Other treats my lot get are nuts and seeds (in moderation), low salt/sugar human cereals (basics/economy cornflakes tend to be both healthy and cheap so they're excellent for treats, and snap up nicely into little pieces so you can get a lot of mileage from one cornflake), and mealworms (again in moderation). I'd say each rat gets several mealworms, several small (5-10mm?) pieces of nuts/seeds, and a tablespoon of cereals (plain puffed rice and basics cornflakes) per week. EDIT: I make my own mix, and I account for all of the above treats when I do my calculations, so I know their overall dry food intake is balanced. If I were giving treats on top of a nutritionally complete mix or nugget, then I'd be much more sparing.
Rats do need salt to survive just as we do, but they don't need that much, and as you noted their tendency towards kidney issues means it's a good idea to watch their intake, particularly as they age. Commercial mixes should contain at least the right amount of salt anyway, so anything more they get would be extra.
Yoghurt and dairy in general:
I'm lactose intolerant (go me!) so I'm going to wax lyrical a bit. So. All mammals normally lose the ability to digest lactose as they mature into adults. (Lactose is a sugar present in almost all mammal milks, except for some marine mammals, for those of you who don't know.) The only exceptions are certain populations of humans who have evolved adult lactase persistance. (Freaks and mutants, the lot of you.
) For the rest of us, human and non-human alike, eating significant quantities of lactose in adulthood leads to gastrointestinal upset: once the undigested lactose makes its way to our lower digestive tract, the gut bacteria there feast upon this unexpected influx of delicious sugar, and produce a lot of gas as a result. This leads to the trademark symptoms of lactose intolerance: bloating, flatulence, abdominal pain if the gas gets trapped and builds up, diarrhoea if you're really badly affected. It's not terribly dangerous, just a bit unpleasant for the end user (and anyone else in the same airspace
A lot of rats probably get some lactose in their diet: yoghurt is not an uncommon treat, as are yoghurt drops. In moderation it's probably fine (even I can still have a little splash of milk in my tea without much trouble); if you're really giving too much, you'll probably know it in a few hours.
Young rats in particular shouldn't have much if any issue with it; I hid medication in yoghurt when mine were still kittens. I have however decided not to give them anything I couldn't tolerate myself now that they've all matured, mostly because I know what it's like and while it's not all that bad, I'd still feel a hypocrite if I did.
Depending on the background of your girls, you may want to give them high-protein things like egg every two or three days for a couple weeks longer maybe? As adults, it depends on the protein content of their standard diet, but once a week is usually plenty. Mine get egg roughly every fortnight – one egg between four just like you – and on the alternate weeks they get fish.
To start off with, there are many articles covering a variety of rat feeding topics here: http://www.fancyratsforum.co.uk/viewforum.php?f=14
For a more in-depth treatment of the topic, The Scuttling Gourmet is an extensive guide to the feeding of pet rats. The latest edition can be purchased at various places I think? including a shop called Rat Rations, a stockist of loads and loads of rat foodstuffs: http://www.ratrations.com/the-scuttling ... -1577.html
Rat Rations have all sorts of ingredients that may be of interest if you want to add to your rats' diet. They also carry some complete mixes developed by various rat-keepers – these need separate vitamin/mineral supplementation, but Rat Rations also carry suitable supplements, including their own DailyRat3 which I use as it's quite economical. (Commercial mixes need supplementation too, but they just spray it on the mix, or blend it into the pellets.) Sit down with a cup of tea before you have a browse, and be warned it's easy to get carried away on there.
Nutrient Requirements of Laboratory Animals is an academic reference which is both free online and has a chapter on rats, and is my go-to for a basic guideline on rat nutrition. Note however that because laboratory rats live a very different lifestyle to pet rats, and are kept for very different purposes, information from the lab cannot be directly applied to the home environment; this reference is mostly useful for getting a ballpark idea of what rats need: http://www.nap.edu/read/4758/chapter/4
Hope that helps and I haven't bored you to tears? (Or spoken too much out of my arse...