Just wanted to post an update for people who are reading this.
So I went with my gut and got a 3mm metric nut from my local hobby store. I took the whole idler wheel off, nut, bolt and all and took it with me to the hobby shop to show what I was talking about. It turned out that the threads for the nut were stripped because I over tightened it. Lefty loosy righty tighty, the saying goes. But I didn't know that to get the nut loosened or off it was righty loosy, lefty tighty. (with a wrench pulling it toward you should tighten it, and moving it away should loosen it. But instead its moving it towards you loosens it, and moving it away from you tightens.
Confusing.) The screw for the idle wheel was threaded differently. The hobby store clerk said that the bolt was also a bit messed up but it should work. So for a five dollar fix I was able to get thing storted and get the track back on and tightened to match the right.
Well, so I thought. So I did a test run and drove it around for a couple minutes and everything worked great. But after awhile it developed this screeching sound and tends to lean to the left track, (the track that I had to replace to idler wheel nut to tighten). So, it sounds like something is borked now, like its either a gear is slipping because I didn't reconnect everything the way it was properly, or over tightened the track. Trouble shooting this has taken up most of my afternoon, and is getting me a bit frustrated. It works, yes, but I am afraid of doing something that will just damage it further just because I am driving it around.
So, I am putting it away and my tools down for now, and my next step will be getting a Mato Sherman RC Tank Metal Rear Idler Wheel with Adjuster.
I have a question for people reading this: will the Sherman RC tank metal rear idler wheel also fit on a Mato M10? I won't think that they would machine two different types of rear idler wheels. Cutting costs and all that.