CAE can get up to 5" so they will not even work for your 30 gal mine grew 1" in a month in a 55 gal tank. For a 15 gal you're not going to be able to put much in maybe guppies, endlers, platies,...... Also I would do just males so you do not overpopulate. Remember the smaller the tank the quicker the water conditions can change. Be sure to do frequent water changes and monitor closely. For the 30 gal try ottos, but they can be difficult to stay alive without proper water conditions. Also cories like to schoal I would suggest adding 1 more if they are pygmies. I had a 15 gal with neon tetras also, that may work.