Well I sort of hate to double post like this, but there seems to be two nearly identicle threads so here goes:
Finally a thread I can really sink my teeth into. Pairs and groups are the BEST way to enjoy many fish IMO, they are just so much more interesting. OK- what I have kept:
The basics, clownfish and seahorses, many species of each. Successful breeding of both and rearing of sh fry.
Mandarins- both species in pairs, no spawning.
Red Scooter blennies- spawning pair.
Flame angels, pairs and groups up to 5. What I would call "pre-spawning" behavior observed in one pair, lost female to open top.
Pearlscale butterflies, two housed together, sexes unknown.
Whitespotted Bamboo sharks, male/female pair.
Chevron butterfly (C. trifasciallis). Unsuccessful at keeping two together, had to seperate.
Jackknife drum (avitar)- group of 3.
Red Faced butterfly (C. larvatus). Behavior indicated male/female pair but not confirmed. Tragically lost to my own stupidity at 6 months.
Orchid dottyback- breeding pair.
Fairy wrasse (C. ventrallis), one male with two females.
Pakistan butterflies, group of three.
Coral banded shrimp, two breding pairs.
Bangaii cardinals, breeding pairs and family groups.
Blue neon gobies, 2's and 3's
Queen angelfish- unsuccessful at keeping two small juvies together, had to seperate.
I get to set up and service tanks for others so can experiment a lot. I find the easiest way to establish a pair or trio is to start with 5 or 7 of the species in a large tank and let them work it out, removing any that need to be protected. For whatever reason odd numbers seem to work better. Starting with only two has failed too many times.