If you have the marui ras, you can fit a small battery, if you have the jg ras, it comes with the navy grip you have pictured and the ras cannot fit any battery.
Aftermarket ras are usually expensive and chunky.
The navy grip can hold either a small 8.4 or certain lipos like the hot power stick as long as the ras has holes cut for the wires to pass.
I have the jg and hated the look of the navy foregrip so much that I now use a peq to hold a KA 11.1 lipo
Go to lipo. No point in stringing batteries together.
Grand Poobah of T.W.A.T.