Long tentacles can get pretty huge. We have a LTA that is about 18" wide when opened!!! it has its OWN 30 gallon breeder tank that is plumbed into our 180, and is hosting two false percs.
we also have a green bta that is in our refugium that is hosting two cinnamon clowns. this is also plumbed into 180
than our true percs in the main tank are hosting a giant toadstool. Its a pain to feed nems in a big tank like that.
in our 120 we actually have 3 nems. a bright purple haitian, a neon green bta, and a pink bta. all do fine.
there are many risks as everyone said with nems tho. Expensive corals could be stung, etc. I've heard of this "chemical war fare"
people talk about when having more than one nem, but myself have not had that problem.
good luck