small size species will work. for instance P. senegalus and P. mokelembembe will be fine in a standard 55. others that might work are P. palmas polli and P. delhezi, although these two get a bit larger than the previous two and do better in a wider breeder type tank.
not sure about your FH question.
ps. just realized you are in hawaii. don't know if you know, last i read bichirs are illegal in HI...
i know there illegal...but there are always ways...dont worry i have seen rbp, asian aro, two types of lungs down here...so there are ways... thanks for the concern though.
IME flower horns (hybrid cich) are too agressive to be placed along w/ bichirs, they are often kept by themselves, so I would have to say no o that.
I had one FH fry along w/ my bichir grow-out tank before, although the fry was less than a 3 inches, it nipped the fins of my 4-5 inch bichirs