It’s two yellows and one blue dice for the fire hydrant bat, and yes, you still roll the regular dice when critting.
Damage however is replaced, so you don’t deal both the regular AND the critical damage. Critical damage replaces the former.
When you look at another weapon card you may see a small “+” next to the dice on the critical section. That means you add the dice for criticals to your normal dice unless it is explicitely stated otherwise.
I see no reason why the Behemoth shouldn’t be able to have the impatient perk and “Light” mod. Go for it!
One thing though: “!” gives you crit tokens, but not quick actions. For quick actions you need a gear symbol.
The behemoth also can’t collect crit tokens unless you make him heroic in which case you already have a good chance to get quick actions from vats.
Also I’m not completely sure, but I think you may not use the same type of quick action twice. So if you get a quick action from vats and another from impatient, you cannot make two quick action attacks, but you could make one attack and one charge for example.