I've heard of there being exceptions to the hi cap rule but don't take my word for it. If you have an odd gun that xt doesn't have magazines for and all you have is a hi cap, that'd probably be fine, but they may not let you fill it all the way so you don't have like twice as much ammo as everyone else.

For any of your questions here though, you'd be better off calling and asking them just to be 100% sure.