long story short 13 players works well
I concur. Although I end up varying how and where my players lineup for training and tactics, I've pretty much had an effective 13 man roster for about the entire time I've been here. (I currently have 14 but the 14th is a fairly cheap draftee of mine, so I don't really look to fire him as he'll pick up minutes once I complete some player movement.)
A trainee getting 48 minutes in one game and being the backup/reserve in a different position in the second league game has been pretty effective for me in GS management. Having good enough backups usually means that I can be confident my starters won't go beyond 36 minutes too frequently unless I really want them to or if someone gets hurt.