I wouldn't worry about the toxicity as much as the fact that they would be predators. The toxicity in amphibians is not extremely potent. You wouldn't want to eat one. But the water will dillute it to the point it shouldn't hurt the fish. Fish eat them all the time. My point above was that they are toxic but meant to put more emphasis on the predation.
A newt may not be that bad. Depending on the size, there is still the chance it will eat a small fish but once the rasboras get older it probably won't be able to eat it. Newts aren't known to be the fastest creatures on earth either. Keep it fed well and it shouldn't be a problem.
One thing you do need to watch is keeping the newt's waste cleaned up. Amphibians and reptiles are nasty animals and their waste builds up fast. Their waste can very quickly foul water and draw little bugs. When I say "foul water", I mean FOUL WATER to the point it smells raunchy.
The ADF will definitely eat fish. Once they get up to quarter to half dollar size, they would make a meal out of a rasbora and would likely injure an even larger fish.