Not all nems are good hosts for crabs... carpets and other more "firm" ones are usually better. I am not saying that BTAs or Condys are impossible, just do not always work out well. Clowns and crabs can coexist in most situations. When I had a carpet, the crab stayed near the mouth and the clowns out into the body more.
As with any symbiotic relationship, what works for one might not work for the other.