A 20C won't fit with the body. It's too long, plus it's heavier. It's also overkill. The AXE doesn't draw that much continiuous amperage.

Yeah you do have to use the rev pol conn, but it's no big deal. The only problem I found with it is the male pins tend to slip out of the connector, but that was easily fixed with some JB Weld. If you look close you can see it in the pic. If you don't want to use the rev pol conn you could physically reverse the balance connector wires on the battery and it will plug directly into the Equinox. Just be careful if the wires short you could destroy the cell.

I also wanted to add which ever way you decide to go stay away from the Electrifly batteries. They're junk and I'm speaking from personal experience. I had one fail after one charge.