The class is for 4-cell NiMh - if you don't want to run that, perhaps you'd be better off in another class?
Like I said, it's just my owm opinion. I race in a open Pro10 class. Just 17t brushed and 9.5t Brushless is allowed at a maximum voltage of 7.4V (All batteries are allowed, Nimh, Nicd, Lipo, Life etc.). the most people run Lipo's, some run theyr old GP3700 Nimh, and me LiFe.We race at very small to big outdoortracks and it works fine...