What is the esc rated for? Looks like it says 2-5 cells on the side of it, but it may be 6? I have never run an mgm, but try to stay close to the mfg recomendations. 18 cell nimh is 21.6 @ 1.2v per cell. 6s lipo fully charged is over 25v. What are the caps rated at? If they are 25v then I would not try it with 6s.