They will eat most small animals such as insects, worms, fish, daphnia, snails, mosquito larvae, etc. so plan their tankmates according to size and temperament. Anything too small will be lunch while anything too large may harass the newts or outcompete them for food. Also consider the aquascape and what can successfully hide in the platns and rocks.
I keep a variety of freshwater shrimp in planted shrimp-only tanks, such as cherry red, tiger, ghost, Malaya, etc and occasionally feed the extras to my N. viridescens to vary their diet. The smaller ones usually disappear after a few days. Same goes for baby guppies and endlers.
Larger animals that are more than a mouthful are usually ok. Every tank I have has a basic clean up crew consisting of fish/shrimp/snails, normally a small catfish-typically a cory, several nerite snails and a few 1-2 inch shrimp. The catfish keeps the nooks and crannies at the tank bottom clean while the shrimp maintain the plants and the snails handle the glass.
I try not to overdo the additions because the tanks are basically setup for newts. I have separate aquariums for community fish as well as a small outside pond that I am always tinkering with, so there really isn't any temptation for me to add other animals to the newt tanks, other than to vary their diet.
