Bummer about the seedlings, but it seemed likey to happen. But, old plants are old news...just don't fry the new batch, lol. Keep on top of their moisture, that light will dry 'em out quick.

I'd transplant when the leaves all overhang the cup. And yes, a 'normal' indoor potting mix would be wise. Most commercial potting mixes are fine, as long as it's ph buffered, and has perlite.

If forced to use soil with time release nutes, go with the one that only lasts a couple of months. Avoid those that last 4-6 months. :thumbsup: