To make a well rounded SF, it makes sense to train a player as a big for a few seasons, then as a guard for a few seasons. I think this is what you are doing with Tellez, right? If so, it would be a great idea to find a second guy that fits the same general training plan.
Later on, the most difficult guard skills to raise will probably be OD, JR, and PS, so I would look for a young guy that is relatively high in those areas. His big man skills should also be decent already (especially ID). That's what I would look for at least :)