CT High Guard is, aside from the character generation chapter, a fleet combat sim. The limitation on weapons, which is explicitly stated in the construction rules by the way, is an artifact of that. DGP recognized that and, by removing the sim's limiting USP, removed the constraint as well.
Well, I'd love to see the cite in HG for that. All I see is requirement to categorize things in batteries, nothing that says I can't have 2 different meson guns.
Perhaps you're thinking of this: "A battery may be as few as one turret, or as many as ten, but all batteries of the same type of weapon must have the same weapon code (USP factor)". I read that as dealing solely with turrets, and keeping the battery sizes consistent across the turrets, not mixing turrets, bays, and spinals.