Oooh, should have mentioned that problem. I found that when I got my RC4WD set the GorillaMaxx single speed coupler was not a direct fit, I had to dremle a fair bit of material away from inside the curved slots before they would push together. Given the choice, I would dremel the Gorillamaxx part rather than attack the gears them selves.
There is one other option though:
http://www.rc-monster.com/forum/showthread.php?t=21856
If you ask nicely, and throw money at him, Im sure he could make you the single gear you require
