The spacing you go for totally depends on how much room you have to allot to this hobby/addiction, and how much time you have on your hands!
If I'm just re-planting them into for example, two-three flats for each flat of seedlings, I'll try and space them about 1-2" apart in each direction. By the time they fill this space, by the fall, you'll be able to see which ones are most appealing and re-pot those into separate pots, or even into a nursery bed outside.
I found they did perfectly well here for me this past winter, outside, no particular care, under 18" of snow. The ones that did the best were in 2" pots, with Sunshine Mix #4. The ones that did the worst were in 4" pots, same mix. Still puzzling over that. I think they got too wet as the polymer in the mix breaks down. You won't have the same issue, as you live in a much warmer climate. Don't be too kind to them, though, they don't appreciate it, and they will die. Be tough, and callous to them. Make sure that whatever mix you use is perfectly draining, whether in the nursery bed right in ground, or in pots. Very crucial!
