I've worn one under my suit and it didn't slide or tear the suit. When you get a brace fitted it is fitted to your leg with nothing coming in contact between the brace and your knee.
I have seen guys do both ways but under seems like a pain in the ass to deal with. You can use smaller pads on the brace and put it over the top. I din't want to mess with that so I use a really good long sleeve spring or drysuit when its colder.