There isn’t an option for them to be their own specialized motorized slot.
The options are either infantry, vehicle or both.
Honestly, they’re better off in a special slot exclusive to itself. The ability to rapidly maneuver and put down rallies is powerful. The fact that the squad itself is armed with SMGs also promote them simply being used to rush the point and dismount.
If in the vehicle slot, non-premium players are unable to bring either a tank or plane, and gets in the way of a team’s ability to fight enemy armor with your own armor.
If in the infantry slot, it’s easily spammed and turns the gameplay more arcadey than it already is due to the motorcycle hordes being a thing.
If it has its own motorized slot, it limits their numbers without preventing friendly tanks from spawning in.
Outside this simple option, a “motorized squad” can become a new type of squad that starts from motorbikes, all the way to cars and trucks. But that isn’t what I’m after. I’d really just say that they should have their very own special slot instead of being in an infantry/vehicle slot.