Before you buy a 75, which is really too small to allow you to stock anything else, you should probably research any fish you'd like to own (i.e., fish you want to put into this tank). You'll probably find that the two fish you already have require larger-size tankmates, and that they require fairly aggressive tankmates. In other words, the 75 just isn't going to hold the type of fish you're looking for. So, research fish, figure out which ones you really want, and then tailor the new purchase to that community. You might find that a 125 is really what you need, in order to fit all of the fish that you want to fit, and this would allow you to keep your pleco (he's not going to fare any better in a smaller tank... seems like this would be obvious). You may as well make sure that you know what you want before you spend your money. Personally, I'd never pay the cash to upgrade from a 55 to a 75. It's a lot of money for not a lot of volume.