Hello.
The potential maximum instantaneous current draw from the Wild Thumper 4WD motors would be around 26.4A (since there are 4 of them: 4×6.6A). This condition can happen when the motors are commanded to drive full speed from rest or when the motors are all stalled. I recommend ramping up the speed (e.g. using an acceleration limit) to reduce the sudden spike in current when starting up, and in practice, it should be unlikely that all 4 motors would stall at the same time. You can still use that value as a conservative estimate though.
If you really wanted to regulate the motor voltage, you could probably get away with powering the motors on each side of the robot from a separate D24V150F7 regulator; however, please note that to do this you would need to drive the motors on each side of the robot from a separate driver, so this wouldn’t work if you are using a single two-channel motor controller like you suggested in your other thread. An easier solution would be to limit the maximum duty cycle you run the motors at. For example, since the maximum voltage of a 3S LiPo is around 12.6V, if you limit the maximum duty cycle applied to the motors to around 57%, it would be similar to driving them at 7.2V.
I am not sure what you are implying when you talk about ignoring the C rating of your battery, but if you use a regulator, it would still be powered through the battery and should still be a consideration. Power is conserved, so the power into the regulator (plus some efficiency losses) equals the power coming out of the regulator. For example, if the motors draw 4A from the regulator at 7.5V (which is 30W), the regulator would be drawing around 2.4A from the battery at 12.6V.
By the way, if you have not read the “Understanding battery capacity: Ah is not A” blog post that Patrick suggested, I also strongly recommend it.
Brandon