Level 4 trainer is fine for your purposes. 21yo min. - 80 total skills, 9OD/ID, 6PS/ISThose are the minimum reuirements for a SF in the U21 program right now, according to this thread. Unlike guards or bigs, a U21 training plan at SF doesn't really hurt any long term goals you may have with this player. MY advice would be to set your sights on those requirements. You may not make it, but even if you don't you are well on your way to a great SF. As far as training another player, most people do train at least two people at a time. As long as their training plans fit together well, that's a great idea.
And why is that hrudey?
Would my 21yro pg/sg or combo whatever be able to be in u21?