Very few commonly available newts (but i dont know whats available to you in the U.S) will have toxins dangerous to fish, and a sponge filter would not make a difference to this anyway.
If a fish was to eat one of your newts then the skin itself could poison it, the same goes for if a human was to try that.
I have kept chinese fire bellied newts, cynops orientalis, with a hillstream loach, which worked well as he cleaned up their uneaten frozen food.
Not sure if i would recommend a betta as they may steal the newts food, or get be nippy towards the newts.
ive never tried this combination myself, but dont think i would advise it.