Remember, Battletech has no rules regarding what faction gets what, it only has guidelines that you are free to ignore as you wish. Between trade, salvage, and theft, it's feasible for almost any given faction to field at least one of almost any given mech/tank/BA. If you really want to stick to hard canon, the Marauder BA is sold to mercs, where it can then spread everywhere. I'd see nothing wrong with a squad or two in the service of any given House.
There are NEVER any paint requirements. Paint em however you want, or don't paint them at all, it's your call.
Number of suits on a base is also 100% your call. The Marauder is a fairly large mini as far as BA go so my advice would be to use one fig per base to represent a squad, but if you want to put four or more on a base, it's your choice, and nobody can tell you you're wrong.