Cant seem to edit this(is there a time limit?).
I apologise if my last post came across as heavy handed, and I certainley didnt mean to discourage you from trying.
What I meant to say, is that this particular combo is a bad idea - I've tried it. I had to seperate the two within 15 minutes. The newt came to investigate the new rock that had fallen into his tank. The crab didnt move for the first five minutes or so, untill the newt got close at which point it lunged for it and settled down again. Stupidly, I didnt split them up then but kept watching. The newt climbed out and retreated to his burrow(a partialy buried cork bark curl against the side of the tank, siliconed to the back wall), the crab followed shortly after. By the time I'd literaly ripped the tank apart to get to them the crab had the newt in both claws, the rear third of its tail was completly mangled and one of its back legs missing.
Despite my best efforts, the newt died 3 days later.
I later moved the crab to a community aquarium, I'd lowered the water level so there was some wood it could climb onto. I'm not sure how it managed it but two days later it had chopped a male betta's fins to pieces, that died as well. It was a viscious little git.
Species mixing is deffinatley possible, and plenty of people have had a great deal of success with it, though it is quite a controversial topic and many members here are against it.
It has to be done carefully, well thought through and most importantly observe the animals closely for any signs of problems before they develop. But it can be done.