I'm going to agree with other people here about not going with the snapper. They get big, and they're aggressive, and they need a huge cage.
And wow for keeping turtles with fish, I had a red eared slider and I bought a pleco for him, and it took him a month before he caught it and ate it.
Red eared sliders are cool turtles, but they need a big tank. They can get to be a foot long (biggest one I've ever seen anyway). They're cool because if the tank is big enough you can keep more than one. Also you can let them go outside and stuff in warm weather, which is fun. You need to watch them though cuz they're fast little buggers. I took my eyes off mine for like 5 minutes and later found him making his way down the alley. Anyways definately do some research. Books are great, there's a book, I think it's called the exotic pet handbook, I'm not sure, I'm in the process of moving and it's at my other house right now. It's a good book though if you can find it, it's green and white and has a praying mantis on the cover along with a few other exotic "creep crawly" types of pets. It basically has a bunch of different kinds of bugs, reptiles and amphibians and gives them a page each and lists care difficulty, if they're ok in groups or not, what type of set up they need, that type of thing.
Sorry about the length, and I hope you find a turtle that's good for you. I've recently been looking at mud turtles and think I might start trying to find some.