I have extensive experience with
Polypterus ornatipinnis (I have 3 of them at this time) so I'll try to answer some of your questions.
Polypterus ornatipinnis, the Ornate Bichir, is the largest of the 'upper jaw' bichirs (those with an upper jaw that is slightly longer than their lower jaw - unlike bichirs such as
P. endlicheri). The Ornate can reach lengths of up to 24 inches or so but it takes many years for them to attain full size. If you get one of these fish you can expect relatively rapid growth for the first couple of years (up to about 10 inches in length or so) and then a slow-down.
In my experience,
Polypterus ornatipinnis is among the more secretive bichirs - much preferring to do its wandering at night and hiding when the lights are on. They are decidedly predatory but not overly aggressive. I personally don't keep any of my bichirs with aggressive cichlids but the Firemouth might be a good match in a large enough tank. As for tank size...if you haven't already bought it, a 50 gallon 'tub' tank would be a better choice for the bichir than a 55 gallon. The bottom area of a tank is much more important for bichirs than the overall length or height and the 55 gallon will be too narrow for an adult
P. ornatipinnis. The 50 gallon 'tub' is a wider, shallower tank and is more appropriate for bichirs.
-Joe
Here are a couple of my
Polypterus ornatipinnis:
