You're absolutely fine to use the 9.6v NiMH in that gun. Butterfly just refers to the shape/style/configuration of the cells in the battery (different styles are used to fit in different types of spaces) it doesn't effect anything else.
Lipo's are more powerful per size of battery than a NiMH. A 7.6v Lipo is roughly equivalent to the power output of a 9.6v NiMH battery, they will have similar trigger response and ROF. LiPo's require more involved care and charging procedures than NiMH. LiPo's tend to be much harder on your trigger contacts because of the high output.