They don't necessarily have to have played in a single game. That is up to you as manager. Also, since we have WW and trades, some may have just been obtained, so, no, there are no restrictions regarding that. And you have a window from the 110 game opening of AAA to the transaction deadline to send people down, test how people you bring up might be doing, or just spot use them to rest starters, and then live with who you have on the ML roster when the transaction deadline hits.