No other fish are likely to bother it. There are some that are even likely to bother your fish, but BTAs aren't aggressive that way.
Aside from choosing the kind that most likely will do what you want (stay put and host clowns), they other thing to be careful about is buying a healthy specimen. Will you be able to see the nem before you buy it, say, from a LFS? Or is this going to be an internet purchase?
Things to look for if buying locally: is the nem inflated, mouth nicely closed up, and foot intact? "no" to any of those suggests "don't buy!". There could be valid reasons for the nem being deflated or having an open mouth, but it's something that you will want to check out.