Li-po batteries are very good to supply large current. I just tested
"Tenergy 7.4V 900mAh, 25C High Power Lipo Pack"
on Nybble.
It seems that with its super power, the protection in NyBoard's step down chip will always be the bottleneck. So it's a must to bypass it as discussed in previous post.
It's also recommended to design a customized battery mount to replace the original one on the belly, to keep the center of mass low and symmetric.
I use LiPo's for most of my hobby projects and have a nice flat 2S 1000mah for my Nybble. Instead of battery compartments, what works well is just a bit of self-adhesive velcro on Nybble and battery. Easy low cost solution and you can even move battery a bit around if you want to play with center of gravity.