Hey Monty, I just joined this fantastic forum maybe five minutes ago.
First of all, I would like to say, thanks for doing your research before purchasing an amphibian. I, myself am keeping fire bellies and they are hands down one of the most awsome anurans. The water does need to be treated with dechlorinater before usage, distilled water works too. If you didn't just be sure to do so next time, it will not harm your toads all the way to death. Second, yes the waterfilters are excellent, I use them myself, the toads actually love having a current, just be sure that the current is not like the Colorado river! And finally, absolutely, you can keep other aquatic specimen w/ the toads, I have done so with my vivariums. The toxins excreted are not enough to harm any aquatic specimen like snails and shrimp. Yeah, the toads might snack on the shrimp when given the chance, but usually the shrimp are watchful and are very fast to get away.
Good luck with the fire bellies! -Shawn