Usually with the starter or germinating mix, you could keep the seedling in there until it forms a few sets of true leaves (some people use 2 sets).

If you find that the 2nd soil mix holds too long, just water with less quantity or frequency (and hopefully you punched holes in the styrofoam cups). Don't transplant too many times in its short life or it may not be able to keep up with the stress.

The seedling may not exactly need compost at this stage but I don't think having it is necessarily bad. I say this because not everyone starts seedlings from a seedling mix -- some use their garden soil and some use the potting mix that they already have, without having to get a specialized growing mix.

Also, for a slightly different method, use those cardboard egg cartons. Since they are cardboard, they will eventually decompose in the soil, meaning that you can cut up the individual compartments and plant the seedling as-is in the ground or pot when you are ready

When you do transplant, just remove the bottom of the compartment so the plant has full access to the new soil.

Thank you! Good info. Yes, I punched holes in the Styrofoam cups. I started some seedlings in the paper type egg carton, but they got moldy - so I thought maybe best not to use those? I don't know if that will hurt them or not. I felt like it couldn't be this hard - dirt is dirt right? At least where I come from it was - we just direct planted into the GA clay. But it's much colder here, thus the reason I needed to start inside. Thanks again for the help.

Please check out the seed starting forum for best info. Garden soil isn't good for seed starting and neither are cardboard or peat pots.

For now, keep soil just moist, not wet.