I rather like the idea of allowing pretty much anything that the hull size will allow along these lines, because that opens up some interesting options for things like defensive installations. A minimum size hull with an outsized weapon, power plant to run it and the necessary electronics/computers, armor, space for ammunition bunkerage, maybe accommodation for crew (or it could be an automated installation)... put this in a buffered planetoid hull, and you've got a vicious defensive SDB-type installation. Note that I didn't actually mention a maneuver drive in there? This could easily be an emplacement, rather than an actual boat. Or you could put some form of drive in there, but I don't visualize this thing really moving around much when it's on station; this is a sentry rather than a patroller.