My Cp/JFB was alone for more than a decade before I found some tank mates for her. There were no problems adding more Cp's to the tank - all animals were long term captives and healthy, and they accepted one another with no aggression (in fact, barely any curiosity). However, I do not feel that newts need companionship. I wanted to breed her, and thats the only reason I got more Cps for her tank. I would not add fish to the tank, or another species of newt. I do know that as far as fish go, white cloud minnows are one of the few if not only fish that can be safely kept with newts, but if you want to add more animals because you feel your newt is lonely there is no need. Caudates do not live in packs, herds, schools or family groups in the wild, and they certainly don't need "friends" in captivity.
Heather