For a 20C discharge LiPo pack, I wouldn't go with less than a 3000mah pack. The 2500mah pack you have listed would be getting close to minimum, plus your flight times would be rather low.

If you wanted to stay with the NiMh packs you would need to get two 6 cell packs and run them in series.

Be carefull with the hatch that you have made. Do you have any other supports inside the fuselage? I know you don't want to fly around and do a battery pack bomb drop.