How many places are there in the tank where the smaller caudate can hide? You should have one peice of structure (i.e. a plant) for each caudate in your tank. Naturally the more dominant caudate will take the best spot. Putting in more hiding spots will cut down on agressive and dominant behavior, give the newts some space.
As for the newt being skinny, you might want to make sure the larger male isn't eating all of the food before the other caudate. That is why I hand feed my newts, because if I didn't then some of them wouldn't get fed.
