The newt should be in a separate tank. Pachytritons, like the vast majority of caudates, prefer cooler temps than most tropical fish. As you stated the added salt may be a problem as well. Another factor is aggressiveness. I know very little about spiney eels however paddletailed newts are very aggressive and will often try to eat anything that moves. Granted that it would not be able to swallow a 10 inch eel but that may not stop it from trying.
Chip