So, if I read this correctly, I'm assuming you got a machined front wheel already? If so, just stick with the front brake when riding it SS "some of the time" and dont worry about about mismatching front and rear rims (M and NM). If you DIDNT get the front wheel machined, it'll still stop with the painted sidewall Just hit the front brake a few times pretty moderately so to get some wear on the sidewall... it'll wear evenly and stop just fine. BUT, it will look crappy. It would be better if you get a properly machined wheel for which you'll be using a brake on.