Literally just buy 7.4v LiPos, they're what everyone and their dog uses and has been using for yeeeeears. How those did not come up in conversation at the store I seriously seriously cannot get my head around, smacks of a shop palming off undesirable stock frankly. 7.4s (very roughly speaking) have the same sort of overall output as 9.6 NiMh, except they're far smaller, far lighter, can be charged far quicker, have a far more consistent power output over the duration of the stored charge and don't need anything like the maintenance in terms of long discharge cycles for storage.
If you can't return what you've bought then fair enough it is what it is, but speaking generally to all readers here, buying NiMh now is like buying DVDs.. maybe VHS tapes.