You wont notice much difference in speed weather you use 7.2 v or 6v
I dont think a 7.2 nimh battery pack would fit on a nitro car. Just get a decent 6 volt NiCd pack. They all generaly use 5 cells so they are 1.2v each cell.
Usually when you buy the pack the shop will sell you a charger with it weather it be 12 hour or some fancy one
If you have a decent 7.2v charger they can charge reciever packs... that is if it has that function etc.