bull heads will eat plants and get to big for a 55. the pseeds might be a little cramped in a 55 as they can get to 10" but take awhile to get there. blue spots and banded sunfish are better for 55s as they are 4". being Enneacanthus they all so less agresive and would open up other tank matesstone cat gets too big for 55 as they can hit 12".
Brindled madtom is probably the best cat for the tank
a group of shinners go well with Lepomis sunfish (pseeds) as they compet well for food and reach a size where they won't become a snack especially if you get young pseeds to start.
flagfin shiner is my favorit
another good one is rainbow shiner.
another is Southern redbelly dace
Blackstripe topminnow are really nice as well and inhabit the top of the tank but might end up as food to full grown pseeds as there more slender.