That's fine if you want, just clearly want to avoid people using the preseason to mine XP before rules are set.
Just tell people not to then, in this case it's not hard to monitor if someone does it. Everyone in here plays in one of the other leagues, so it's not like it'll come as a surprise.
I'd definitely suggest turning them off. Especially since everyone has a brand new team, it would suck to be trying to learn your new players and then lose someone to injury before you even get a chance to use them in a game that matters. Obviously that's more realistic, but it's also less fun.