Hello bud
Your little ones need to have 2-4 established leaves before transplanting, once they have this you know that they have a well established root system. Keep your rockwool damp at all times but not sodden wet, if unsure lift pot upwards in afairly sharp motion to remove excess water. As for long stem when you plant them in main pots you can burry them up the stem a little to keep them shorter. Sounds like your doing fine. Keep them warm and youll be all good. Start half nutes when you transplant.

I am new to this myself and am exactly where you are, I use a areo propergator which works fine and keeps things at a good temp. I also have one with long stem but am fortunate that I have someone who is the bees knees to ask when it comes to not knowing things.

Best regards
computec48