When looking for a trainee, I generally look for the highest total skill points guy within my budget. After that I think its important to look for guys that have high skill points in skills that are slow and difficult to train. For me that means that IS and ID are big priorities. These are the two slowest training skills on short players so I would always to try to find guys with 6-7 IS and ID. (or SB if you are one of those guys). The more secondaries on your trainee the less time you will have to train a slow skill with the add bonus of actually playing him at guard more.
The other two skills that you should look for are OD and JR. They are the two slowest training guard skills. In the perfect world, there would also be a big gap between DR (high) and HA (low), this lets you training alot of 1v1 Guards for DR while maintaining future salary efficiency by limiting HA.