I guess I didn't understand the question. You asked if 6s is too much power. The answer is yes and no.
Yes if you run a fast motor, no if you run a properly matched motor and gear right. HV is always more efficient than high current, so if you can utilize the 6s, then I would go for it if you can.
The only way I can see the big 6s pack fitting in the buggy is if you take it apart and re-wire for dual 3s packs and put them on each side of the CD. Of course, then you'd need a mount that lets the motor run in the center and over the CD. A truggy has a lot more room in the body so that's ok.