Creatures to put in a half water / half land based paludarium? Problems finding answers to this.?

Today I'm going to local fish and reptile stores to ask questions. But I want to have the basics run down before I go. We will be building this out of a 20 gallon tank, our apartment's regulations. But we will be building up from it with a tight chicken wire, plexigl, or mesh screen. We know there will be about 15-17 gallons of water in the paludarium. Currently I have fish that will be moved into the paludarium. I already have a filter, and the tank and will begin building shortly. I have a black skirt tetra, and will be adding about four or five more to make a school, and I have two guppies that will be moved into this tank, which we will also be getting about two more of. The only problem is my apartment has regulations on aquarium plants and reptiles. So I need a type of amphibian and will be using fake plants. Our budget for the animal right now cannot exceed twenty to thirty dollars. Now I want a frog but I've read that some eat fish, and some excrete poison. With anyone who may know information on matchings for my fish and a good amphibian that won't be likely to eat them, I need answers.