Ah, the rules. Same as the Grand Titan firing nukes, when none exist in the game for that calibre.
I would assume in vehicle mode it actually pilots somewhat like a tank. Of course, in mech mode it would not.
Speaking of which, I am not sold on the idea that those would actually make such great tanks. Yes, they have definite advantages, but all that left over weight easily eats up the savings on a heavier engine, on slower designs at least. As long as one does limit oneself to ammo-using weapons, heavy vehicles can carry vastly more firepower and armour than these things.
So going off a wild tangent based on my above analysis, QuadVees play to their strengths best by using a high rated engine, thereby A) reaching speeds where the extra movement on pavement/in wheeled mode would incur a significant cost as an engine upgrade, B) benefit compared to vehicles that pay extra tonnage for their engine, and C) allow room for the sinks required for the energy armament that vehicles can't easily have.
It seems to me this makes heavier Quad Vees unlikely, though an 80-ton wheeled assault would be possible.