I know from the umpteen different people ive asked about stock levels theres a huge variance in opinion, but I've got a 30 gal
tank with a 10 gal
sump, and with skimmer + 45lb live rock + plants I my nitrates rise so slow i only need to change water once every month (hopefully never(?) once I get my mangrove trees this week and the current plants grow more)
Thats 5 fish in a 30 gal
, and works out to roughly 1 inch of fish per 3 gallons. "The New Marine Aquarium" by Michael S. Palette says a good rule of thumb is one inch of fish per 2 gallons (pg 89) with live rock. (I know my fish will grow so before anyone jumps on me, im goin 120gal later this year =P) Others will say 1 inch per 5 gallons is too many. From what Ive seem to find its more like... if your fish are constantly pissed at eachother and your not needing to change water every 3 days, your good.
as for the basslet/firefish fighting... they go look pretty close in body shape (and possibly color depending on what you get.) Site ive looked at seem to say the basslet tend to be more pugnacious whereas the firefish are more docile... so it may get picked on, or at least have a hard(er) time feeding. But then again they are smaller (relatively) fish and might be just fine in a 55gal.
(ps the '?' next to the never (above) indicates might not be correct on that heh... detrius for example... but its kinda besides the point anyway)