Species in a paludarium? A Half-aquarium environment and half-land environment.?

I have plans to start this. It will be in a 20 gallon tank, due to my apartment regulations. Currently I own a Betta, two guppies and a black skirt tetra, and yes before you tell me they don't go together they work well for right now. As soon as I get the new tank I will be turning it into a half aquarium/half land environment with the whole set up. So I am wondering after a 33% land and 66% water setup which is what our plans are, or we will extend the top with a new top to make the land larger depending on what species we get. What I need to know is what can be housed with black skirt tetras(which we will be getting about 4 more of) and guppies(again we will be getting at least 1 or two more of) in the water. Also what I could house on the land above water without them eating my fish or killing them.