There really aren't any, only Support Vehicles need to track tonnage for crew.
Not exactly. Small support vehicles have to. Medium and large support vehicles only do it for crew over the maximum or when they're adding quarters rather than basic seating.
Combat vehicles do not have rules to cover any of this. Given that their crews usually run smaller than support vehicle crews do, I'm not sure it's really a significant issue. If you want to add seating and feel like tracking it, it's 0.075 tons per seat; without fractional accounting, that's 6 per half-ton or 13 per ton. (Infantry, which needs room for weapons, supplies, and other things, isn't quite that generous.)