I'd suggest paralleling the motors, and running 4S. Unlike brushless motors, with half the voltage going to the motors in series, you'll also get considerably less torque as they can't just pull all the current in the world to make up for the low voltage (like BL); I learnt a lot about brushed motors way back when I was running brushed setups in my old Wild Dagger. If you want lower RPMs, I'd just gear down personally. On 4S, you'll be running them at their most efficient too.
The MM should be able to handle them easily in High-Power FWD-only mode, but might have a bit of a hard-time in reversable mode (at a guess). Just my 0.2
