Potting up: when and into what
I have some two week old seedlings that are kicking butt right now, and was wondering the best time to pot up, and what medium to move them to. They are currently in seed starter mix (coco fiber), and many of them have 1-2" long true leaves already (one has actually begun its 2nd set of true leaves, which blows my mind a little bit for only 2 weeks). They're in perforated 6 oz styrofoam cups currently (1 each), so they have a little room to grow. Most of the commercial potting mixes seem to be high nitrogen (12-6-6 or 10-5-5)...which when it comes to tomatoes doesn't seem right, but then I wondered if early on the nitrogen would suit them well until they get big enough to make fruits. I'm really tempted to just make my own mix of perlite, peat moss, and the 10-5-5 potting soil...but before I go down that path, I thought I'd get some ideas.
What are some of your opinions on when to pot up, and what medium you use? Or do I even need to pot up (hoping to transplant in 3 weeks or so), considering the 6 oz cup is bigger than most cells I'd buy at the local Lowes or Home Depot.