When I was playing with an 18 servo hexapod, the servos that I had access to ran on 6v. I used this SBEC and a 3s LiPo battery.
To do it again today, I would use a 2s LiPo directly to modern servos that can run on 2s.
This would greatly reduce problems with power supply. Simply pick a battery with enough capacity to deliver the needed demand.
Do be mindful of the limitations of the torque of the servos.
It is very easy to exceed the torque capacity of the servos that bear the weight of the hexapod.
In a 3 and 3 gate, where 3 legs are in the air at the same time, only 3 legs bear the weight. Depending on the length of the legs and the overall weight of the hexapod, the 3 servos bearing the weight might not be sufficient.
In early iterations of my walker, it could easily do a 6 leg pushup from the ‘home’ position with the belly on the ground, but when it started walking, it could not maintain the stance and sank down low enough that the lifted legs dragged on the ground. I bought 6 higher torque servos and was able to continue development.