[bettababy]

I'd nix the crab idea, and the fish idea too. African clawed frogs get very large, about the size of a softball. They are very dirty and grow fast, and will eat anything that fits into their mouths. Watch the newts when the frogs get large, and keep up with frequent water exchanges. Adding a submersible filter will help.
The other thing that should be considered is that newts and frogs are both tropical, thus should have a heater, too. Mid 70's is best for them. Newts will need land and water, and tend to spend about equal amounts of time in them. Be sure the tank is well sealed, as newts are wonderful escape artists, and if they get out, they dry up quick, thus they die. African clawed frogs also tend to jump and climb, so well covered with these guys is a must unless the tank is extremely tall.

 

[bettababy]

Many of the species of amphibian with toxins in the skin are not toxic in captivity. Most of those toxins come from the natural diet that these animals eat in their natural habitat.
I do agree that mixing them is not smart, as most are cannibalistic, too.

 

[bettababy]

The dart frogs are a good example of what I was referring to. In the wild, they are toxic... due to their diet. In captivity, they are non toxic... due to change in diet. The toxin in the wild dart frogs comes from ants that they feed on, if I remember correctly. I'll have to look it up, but I do remember that the species of insect they feed on in the wild is where the toxin comes from.