2 things: first is that the "penalty" for training grey skills is also a "bonus" for training white skills. Adding positions makes skill points distribute more evenly, but overall skill gain or training efficiency doesn't rise. For example, a DC/DL will (basically) never be as good an aerial defender as a DC, because training Defending Corners or Set Piece attacking drills will add points to Crossing skill that is low TEF grey for the DC, white for the DC/DL. I just suggest being thoughtful about multiple position characters; I also don't believe there is significantly demand for them in auction to balance the green pack cost.

2nd point, if you've got the time and resources, consider letting a player drop to under 40% quality before training extra positions. One benefit of buying young and having a team of veterans. Those young players will also become more balanced towards the white skills, so perhaps better at their position than another character of same Q value but more even between grey and white skills.