Actually you could keep the waterfall and use one of these.
http://www.drsfostersmith.com/produc...Ntk=All&Ntx=mo I have seen them in just about every pet shop. One of my local shops actually has a tank similar to yours ( minus the waterfall) that has FBT and one of the turtle docks with various stones in it. It also has some small tetras that are fairly inexpensive.
Also its not so much with compatibility in your case, but also you have to take into consideration the size of your tank. You could very easily mix species in like a 55 gallon tank but in a 20 gallon your choices are very limited. I hope you understand. Its better to have one happy frog than numerous unhappy, sick, uncomfortable, and maybe even dieing frogs.
Also if you decide with FBT you will need a basking spot created by the use of a heat lamp. You could have about 3-4 of them in a 20 gallon tank. It should be approx 50/50 land water, which yours pretty much is. Also when you buy FBT make sure they are close to the same size, as they will eat FBT that are smaller than them.
Cory