Hi Amber. Betta's are interesting, but I wouldn't say that's impossible. First and foremost, I will just say that I wouldn't recommend it. I've heard of cases where it worked out okay, and cases where it doesn't. Bettas are so strange because they have a ton of personality, and they are all different.
I tried to keep a ghost shrimp in the bottom of my betta tank, and the betta constantly attacked it until I (quickly) got the shrimp out. I'll note that I don't mind feeding ghost shrimp, but being picked to death by a tiny betta mouth is a long painful death.
So I took the shrimp out.
I had a betta before that left the shrimp alone.
So, monitor the betta. Does the betta seek out the newts and pick on them? Or, do your newts seem stressed? Are they *always* hiding somewhere?
If the newts and the betta are active, you might be okay. It's important the remember that the betta could change its mind one day when you aren't home and decide to pick on the newts. It's a risk you will be taking.
I'll also mention here, that 55 gallons is HUGE for 2 fire bellies. Are you sure that's what kind they are? That's totally awesome! Just so you know, you can feel free to add more of the same species of newt if you wanted to. How is your tank designed?