Lipos and deans is what you need! And a 7.4v (like 8.4v nimh) in ur stock is all you need!
Lipos are safe I used to use them in RC racing! I lost count the times my Nimh race cells exploded, once one almost shot of the pit table shelf in to my face! Never had an issue with a lipo! They are light, hold charge for months, unlike Nimh that discharge a few % a day, they can be repeaked at any point and don't require a full discharge! Where as Nimh although they don't see a memory effect after being fully cycled if u just keep repeaking wih out a full discharge your voltage will drop so as an example an 8.4v could drop to 7.4! Also lipos last a lot lot longer before degrading, you get way less charges out of Nimh!
With out the right equipment (I had about a $1000 worth) its hard to keep Nimh in good health! Go lipo!
Deans are old tech in rc, as when we used Nimh we just soldered the packs on each time as that way we limited power loss through the connectors! Then when lipos came in we moved to banana plugs (gold type) or deans! Now in racing direct soldering and using no connector gave you a tenth maybe off your lap! In Airsoft they are no advantages to trying to remove or improve your connectors above standard deans that is! (dump the tamiya)
So lipos and deans! Better and more cost effective in the long run!