hmmm, well its not recommended since it probably wont reach its full potential, but if you do your weekly water changes, have a strong filtration, and feed it a variety of nutritious food (tilapia, smelt etc.) then it shouldnt be a problem. but for all upper-jaw bichirs, a 4x2x2 or more is recommended.
if the palmas polli is the only bichir in a 55, then maybe it would be fine for life. but i strongly recommend you upgrade now or in the future.