Most newts, including Chinese firebellies choose bodies of water where there are no fish present. Sometimes even the chemical traces of fish in the water will cause the newts to abandon their home and move to pastures new, I know this because I once ran out of dechlorinator half way through a water change, so I used a bucket of water from my Koi pond, big mistake! Within minutes most of the newts were freaking out and trying to escape, I had to do a 100% water change the next day with purified tap water before they'd even begin to settle down again. Sorry for the long winded answer but it proves just how much newts hate fish!
Most crabs I can think of have different temperature needs to newts and have way too much weaponry for my liking, so forget them as well.
Shrimp on the other hand are usually fine with newts in my experiance, I keep Japonica and Cherry shrimp in most of my tanks, they are fantastic scavengers and the cherry babies are a handy snack for the newts.