I would DEFINITELY NOT put a newt in with anything other than another newt of similar size. Reason being, most newts and salamanders are actually quite poisonous. If your fish decides to nip the newt, it could easily be poisoned and die, and you'd never know for sure what happened. Plus, I'm pretty sure that newts are supposed to be kept at room temp, not tropical conditions like bettas.
Additionally, newts have an interesting life cycle - there are larva, efts and adults. Larva and adults are aquatic, but efts are terrestrial. They don't like water much at that stage, so you'd have to have a half-and-half land/water setup if you wanted a newt in eft stage. I guess it depends on what kind of newt you wanted to get, but anyway, I wouldn't keep one with a betta.
Robert Falcon Scott - Red salamander HM plakat
Passed on- Friedrich "Freedy"
Four African Dwarf Frogs - Dorkus, Sporkus, Baron(ess), and Bubbles!