I have a couple Pulse Ultra 2550mAh 2S RX Lipos. If you wanted something lighter, the 1300s will work. Just fewer flights between charges. With current field charging gear being what it is, and the expense of those small packs, I plan to pick up a couple more and always have some ready while the others wait their turn for the charger.
I would think mounting will depend on the COG of your build. I know on my E750 I had to place on the top of the flight pack to get the nose down.
There are sever nice places on the frame sides to strap a pack if needs be.