A long time ago when I was first getting started I too was told that WCMM's would work with betta's. I was told that having three or more would protect them from getting picked on, that the betta is a slow fish and they are fast so if he tried to go after them (which he did) they could swim away quickly (which they did) although I was never told that they were cold water fish or any other differences. So, I bought three to go with my first betta and they all did really well. I actually attribute their presence to my betta's long life. He got very depressed after I took him home from living at work with me for a long time, I thought he was going to die so I got them and he perked right up and lived for a few more years. Now, I had them together but I'm not saying it was the best situation for them. Now that I know more about bettas and fishkeeping in general I can't say that I would repeat it.
I have also had bettas with snails, ghost shrimp, a cory (I used the catch he and the betta laying on the bottom of the tank together all the time but I'm thinking that's kind of rare) and cherry barbs. Again, now that I know more I would not repeat most of the scenarios and if I did it would have to be with a totally different set up than what I have now. Currently all of my bettas live separately and I got myself a new, bigger tank so I could have more and different fish to cure my "community" cravings.