You've obviously hit a touchy subject...lol
The true answer to your question is not regarding the size of your tank. Being a 155, you have the linear swim space a tang will need. Now it comes to the actual selection of your tangs. The truth is, if you have tangs that look too like one another in body shape, they will quarrel. The fact that the yellow tang and the sailfin have not constantly fought may be due to the large tank size. Only just...
In reality, there are about 5 different families of surgeonfish/tangs. If you keep a fish from each genus, the odds are highly unlikely they will fight to the death. In my tank, I have a Desjardini Tang, Koles Tang and a Powder Blue. Sometimes they show their barbs at one another, but they never fight. It's to be expected with fish seeking dominance in a confined space. You will need to make sure the new tang is at least as large as your largest tang in the tank already. He will be picked on if he's small...
I recommend you pick up a copy of Reef Aquarium Fishes by Scott Michael. It will really help you with your fish selection.
