Generally for beginners, you'd want to train either guards or bigs, and have veterans fill the spots of the positions you're not training. For you, I'd recommend training guards (that would include playing some of your short forwards as guards, since they'd be able to train guard skills quicker and tend to be nice small forwards). Then, you can shore up your front court with another good center. That way, you can train guards every week, and field a good enough front court to win as well.