I have a six line wrasse (among other fish) in my 29G biocube with a collector urchin and don't have any issues. Like was said above it really is going to depend on your fish. Mine will have a little territory war with my royal gramma now and then, but that is more huffing and puffing than anything, never had mine go at it real bad or anything. I really don't see it picking on an urchin, but that's just me. I see you posted that he isn't bothering anyone, so just keep an eye on him, but I doubt he'll be a bad fish, only time will tell though.