My Dels are probably the most active in the tank. They are usually the most in your face too, every once in a while they get spooked and kinda freak out a little but it passes quickly. Only reason Ive said might not want to is a 55 is only 12" front to back its not ideal but like others have said they should be fine. Sens are a definite poly for that tank most "cost effective" and readily available, active, interactive.
Whatever you decide get a lid/glass top and be careful in choosing your decorations. No small holes for polys to get stuck in.