Yes you can overtrain players, I have a Center that was at 100% according to buzzer-manager and got him a pop in IS (he's got superstar potential but 138k salary and now 63 inside skills). It's just terribly slow.
I'd only overtrain if you got a free training slot and are not planning to get a new trainee, and it doesn't hurt your game performance. Otherwise it's not worth it IMO unless you really want a NT player (which doesn't seem like the case at allstar).