Smart question. For years all my seedlings went into
Promix with 10-15% worm castings. That's it. Feed nothing else until you have about 4 leaf sets or so.
Some of the other premade soils can be way too hot for babies. As far as other plants I followed Subcool's recipes, and made soil in bins, and turned and watered it for 4-6 months before I ever used it. Also much cheaper soil using base ingredients like 3 types of bone meal, kelp meal, epsom salt, blood meal, etc. You get the idea.
I don't do soil much anymore, those bins got heavier as I got older.
Look up Subcool, he's got great soil ideas. One time I accidently dropped a toothpick in a bin, and in a month I had a small maple tree. ;)
Good luck,
RR1