You could probably keep the P. p. palmas in a 55g for life without problem, though it would likely feel more at home in a 75g. I don't think you'll have many problems with keeping the palmas with rope fish, from what I've heard they aren't as aggressive as say ornates are. But as anyone will tell you each specimen is individual and will have it's own personality. As I said it should be just fie in a 55g as long as you don't over crowd it. I would suggest limiting the stock list to the palmas, the rope and MAYBE a couple others. The biggest thing to consider will now be your filtration.