yup you can use that battery
providing it can handle the 50 amp discharage rate the esc requires please check that asap. if they can't it will result in a murdered battery, or worse a fire.
when esc's & motors talk about voltage in lipo terms and your using nhmi's just compare the lipo battery voltage to a nhmi, '2s lipo =7.4v ... thats equivalent of a 6 cell nhmi at 7.2v, etc..
Only issue with nhmi's is that most cannot handle the discharge rates required, hence why i stated ensure you have one capable of 50,
most 7.2, 8.4's nhmi;s can only discharge at 30amps max,(most are still old school designs for rc cars) which is when your using brushless esc's over 50 amps i will tell you to use lipo's, and normally 2 or 3s's
only issue is some higher discharge nhmi's they can be expensive, more so then a cheaper 3's lipo which is where you have to thing budget, any nhmi battery that costs you more the £25 and your running brushless i would now say just get a lipo as they are better overall and more ''zoom'' for your cash and in the same price bracket as the nhmi.
hope all this is helping, i'm starting to hate the sound of my own voice in my head