first of all, the rosy barbs keep to themselves and the firemouth leaves them alone. rosy barbs don't get too nippy though in the first place.
the tiger barbs though, different story, but not necessarily bad. (i have 6) when i first put the firemouth in, they were very curious and chased the firemouth around a bit, nipping at his tail. after about 48 hours of mostly hiding the firemouth began to venture out a bit more, and since has established his authority in the tank. the tiger barbs are less active and pretty much stay away from nipping the firemouth. now and then they'll go in for a nip, then decide it's not worth it, turn around and run.
all in all, i'd say they have a good relationship. when the firemouth gets bigger i dont' know if things will change, but so far, it's ok.
the tiger barbs are definitely less active and really calm down when the firemouth is lurking around their favorite plant cluster. kinda like coming around to check on them.
- kip