If you run the calculators, most 55s hold less that 55 gallons of water, if you get a 75 and use 55gallons... Fill with 11 full 5g buckets it'll be a couple inches shy of the top, this is good for a couple reasons.
-Lower water makes it harder for the occupants to escape, a problem with Bichirs and easilly spooked fish!
-Longer distance for your current HOB's to spill into the tank, increasing O2 levels.
-White noise to help easily spooked fish cope with outside noises.
The better benefit is the increased foot print for the fishtank, a 55long is not very wide, and could be easily tipped by friends. If all else fails the 50 breeder as you mentioned which is just a 'standard' 50G is 36x18x18. This might be a better choice for this same reason...