It’s possible in a larger tank if added concurrently but almost certainly not in a 55.
Adding a single tang to that environment is not likely to end well. They can and often to gang up on the newcomer. The only chance you have is probably an acclimation box. A Kole added to a Tomini that already shows aggression to newcomers is not likely to go well -- they're both bristletooth tangs and in direct competition for resources.
I would not add any tangs to your situation due to crowding and the likelihood of the two resident tangs not accepting the newcomer.
If you were to add more tangs, they’d have to be those that stay relatively small and add two more rather than one. No guarantees this would work, and it doesn’t account for space — but a purple and scopas, or purple and black, or purple and gem would give you better odds as aggression would be spread and the yellow wouldn’t choose one target over the other (likely) because they’re all of the same genus. I have mixed zebrasoma in my 180: yellow, purple, gem, and desjardini sailfin. I have mine packed full of tangs but a lot of time and preparing went in to that as well as adding them in batches.
Indeed, same length for swimming, but volume is VERY important particularly in regards to bioload and O2 capacity. When we are talking about a species of fish that will likely reach a minimum of 6in, volume really does need to be considered not just length of the tank. Which in the case of comparing a 90g to 55g actually makes a pretty big difference. I was able to pack 30+ African cichlids into a 5ft 120g tank, but with saltwater, I could not even consider that due to O2 saturation of saltwater compared to fresh. So, one could argue that in the case of saltwater, it becomes even more important to consider your tank volume when planning your tank inhabitants.;)
My tomini is a fish-killing monster in my 5' 150g, responsible for the deaths of at least 5 pre-established non-tang species, suspected to have impacted more, and has to be rehomed.
