I've been running Intellect 2S 30C 5000mah hard case lipos in my Slash with a Castle Sidewinder/5700 combo. Gives me about 25min of run time, fits the batt tray almost perfectly.

Hard cased lipos are made to same size as a 6cell pac so that would be a good way to go and would be more rugged and less prone to probs running offroad. They have wires coming out the top side of the lipo (as opposed to teh end or side) so just be sure that won't be prob. I'm not familar with a Rustler