Hi,

Nicads are just too heavy. You wont be able to hover it on a 7 cell pack on a GWS power system. Also, the GWS motor will burn out quickily on anything above 8 cells, including 3 cell lithium packs.
What I recommend is that you get a pack of KAN 650 NiMH cells. These are lighter than a 600 size NiCD pack, and these cells put out good power. Also, if you want to push these cells, you can charge them at 2C (1.2amps), but a cooling tube of some sort is recommended.

Stefan