Tuesday 2/27 is my delivery day for my first chopper an Axe CP.
I see that some use a TP 1320s, What is the maximum battery that can fit in the front compartment and still use the body and won't throw off the front/back balance. Would it hurt to use the space from the main motor to the base of the tail boom say something like a eletrifly 1500 or smaller?

I think the TP 1320 is about as big as you can go in the O.E. location. I'm using a DN Power 1350. It's about 69mm at it's longest point (including wires) and it does slightly interfere with mounting the body.

Also if you do mount a battery in or below the frame try and get it as centered as possible under the mainshaft and it will minimize cg issues. I personally wouldn't do it because of the added weight and the added labor of switching out two batteries after every run. I am curious to see the length of the run times though.