The mm can't really handle more than 4s, so you can't have those batteries in series (6s) with it.

A 1/8 buggy needs at least 4s to have decent performance. To attain the same speed with 3s, the system would draw too much current and things would become hot fast, so those batteries in parallel (3s2p) is not an option either.

MMM 2650 is not meant to be used with 6s.

Basically, you need to get the MMM 2200 to use 6s. The buggy would be crazy with this setup; reaching 60mph is easy and acceleration would be almost uncontrollable. It'll be fun.

Hope this helps.