One channel controllers are fine for combat robots. The electronize controllers I use all the time are single channel. You just need to have at least one per side of the machine. Stay away from ebay controllers unless you know what you are looking for as the ratings given are meaningless.

A dimmer switch wouldn't work as a speed controller and would quickly burn out.

I might have a couple spare electronize kicking around. I'm planning on having a proper clear out of the parts I no longer use in a couple weeks so keep watching the for sale section.