When you say "above concrete toilet and shower", I assume you mean that they are on a platform of some sort. If you want to determine how much available altitude you have under your slab, you should be able to do this by checking out the nearest cleanout. Wherever your main sewer pipe leaves your house, (if you are on a municipal sewer it will likely be in the front of your house. If you have a septic system, then it can be anywhere) there should be a 4" plug that you can remove (this may not be easy, especially without a large pipe wrench) and peak in with a flashlight. The main pipe should run horizontal and the cleanout should be on a 45 degree angle running with the flow of the pipe. This should give you an idea of how much space you have available.