I would definitely recommend getting yourself a copy of The Scuttling Gourmet if you're looking to make up your own mixes; it covers all aspects of rat nutrition and feeding in a clear and concise way and will answer a lot of your questions:http://www.nfrs.org/sales.html
One of the main advantages of making up your own mixes is that you are in total control and so you can tailor what you put in them according to the needs of your rats. I'm hoping to take the plunge and start mixing up my own food soon as well, but as a good compromise (and if you're looking to move away from nuggets) the complete mixes from RatRations are excellent. I've been using them for a few years now and the rats look great on them (I would avoid the No7 though if your rats are inclined to put on weight easily, and you will also need to give a supplement such as Calcivet/DailyRat3/Calciform a few times a week to cover their calcium and vitamin D requirements)
Pootle, Perkin, Bramble, Teasel, Marley, Oswald, Monty and Roger
Fizz, Cuddles, Bubbles, Horace, Ollie, Humphrey, Fergus, Max - "All the darkness in the world cannot extinguish the light of a single candle"