yes you have it right. So for clarification
Fast, open topped vehicles allow you to
Stand still, shoot everything.
Move 6in, shoot everything, passangers may shoot as well, but only as if moved (so no heavy weapons, rapid fire where needed)
move 12" shoot 1 gun (and all defensive weapons), passangers may not shoot, but may disembark to fire. they can also assault as per open topped ruling. Remember as per FAQ poison is a defensive weapon, hence venom can shoot its cannon and rifles (or second cannon)
Move up to 24 (36 for jet) and get a 4+ coversave. May not fire.
Arial Assault overrides the normal rulings, so ravagers and jets may move up to 12" and shoot everything.
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