My deal is:
Got a VERY fast trainer?
GK: train one-on-one stopper. Then push him until 7* if you can.
DC: train defensive wall SA. Then push him until 7* if you can.
ST: train one-on-one scoring ability. Then push him until 7* if you can.

The reasoning behind this is if you start pushing him to 7 stars immediately, as soon as he gets near the 7th star, he will become a VERY slow trainer and will gain very little.

In the long run if you got a fast trainer and started with the SA you'll only benefit.