I have threadfins and I love 'em. They do do better in a species tank, but as long as the other fish are very quiet they should be fine. Guppies get a bit overwhelming for them though--mine were much happier once I took the guppies out of their tank. I did have a dwarf puffer in with them with no problems, but other people have reported issues. Neither of my dwarf puffers have ever bothered even the quietest fish--they're far too busy with snails!
My threadfins don't seem to touch even the tiniest shrimp, either. They're picky eaters with very small mouths and even smaller throats, so keep that in mind. Totally worth it, though, especially if you can find really nice ones!