Well looks like im late, but Paddlefin newts are about 10 bucks around here and they are amazing.
As for fish Im not a tetra person.
If you were looking to do an amphibian tank 2 African Dwarf frogs can Live their lives in a 20 gallon. They are amazing, really neat animals. Theyll eat anything, but thrive off live food. if you get 2 when they get larger theyll eat 2 dozen minnows or small golds a week along with a healthy amount of sinking shrimp pellets.
A pair of Leopard frogs would be good with a newt.
Ive kept Axolotls before, and Ive never seen one 12" long. I also could not get them to eat fish for some reason. They would only accept crickets and only live ones. Actually bought them from a bait shop for like 2 bucks a piece. They were being sold as catfish bait. They also sold Salamanders, frogs, crickets, golds, shiners, minnows, and theyd get other "catfish" bait in from time to time. Now I cant find an Axolotyl for under 60.
As for crabs, theres a huge selection around here, They WILL escape I dont care if you duct tape the lid closed theyll find a way out.