Need an addition to fish community in 55 gallon aquarium and decorating advice?

The tank just started cycling, so I'm trying to plan what the finished tank will be. I'm using Tetra Whisper ex70 filter, have an airstone in the middle of the tank, and have a 200 watt heater. The tank is long not tall. So far my thought are 10 Zebra Danios, 6 Cory Catfish, and 1 Red-tailed Shark; however, all of these fish are either top swimmers or bottom dwellers. So, I need a suggestion for a mid-level swimming fish. Also, how should I decorate the aquarium (preferably things that I can get cheap). Currently I have gravel underneath of some river rock and glass rock as substrate. I have only a few fake plants in the tank so I would like some live plant suggestions as well. As I said, I need something cheap, so I went into a creek and obtained several large rocks, boiled them, soaked them in vinegar to make sure they were water safe, and used some silicone to make a small cave.