IS-Standard for the LC, IS-Standard & the Cavalier for the FS.
CAN you use others? Sure, but, stuff like the Infiltrator & GDL suits have much better uses than being a Marine.
I think most probably just used the Marine Combat Suit if anything
Basic Marines would use that, but Battle Armor should be in some kind of BA & given the Era. ISS seems most likely to me.
Avalons were likely carrying RCT troopers in addition to their own compliment so there is that.
I tend to be a proponent of just using space marines.
An RCT carries a minimum of 5X the BA that a single Avalon does, so I doubt you see RCT troopers in those bays.
I would think the Avalon has it's own compliment just for defending the ship that won't ever be deployed to the ground.
RCT BA is likely in basic Foot Bays of some other DS w/ BA Suits themselves taking up Cargo Tonnage.