I have an orange back wrasse and Mckosker wrasse in a 90g tank , along with a couple clownfish, yellow tank, pjama cardinal, and coral beauty. I'd like to add an exquisite fairy wrasse. I've looked at the chart and they are not in the same box together, but unclear if they are 'far enough' apart. Would the exquisite fairy wrasse to be ok with the orange back wrasse?
IMO there is a high probability of aggression.
While I haven't had an orange back (Cirrhilabrus aurantidorsalis), per "the chart", they are, on average right in the middle of the aggression scale — of the 5 colors they are orange, or color 3. And, you have it in the absolute
minimum size tank that's recommended.
In other words, it is likely right on the edge of feeling cramped for territory and food competition from other similar planktivores ... particularly other fairy wrasses.
I have had an exquisite fairy wrasse (Cirrhilabrus exquisitus) in a 65g. They're ranked a green (or a 1) in terms of aggression. And yet, when I tried to add a lubbocks (Cirrhilabrus lubbocki) and a pintail (Cirrhilabrus isosceles), the exquisite hounded them both to death ... even though they were all ranked very low aggression (green, or 1 of 5) AND their complexes were all very far apart on the chart.
Granted, my particular exquisite may have just been an oddball jerk...
But the more likely reason is that my 65g was very close to the minimum tank size for that fish (55g) and it felt too cramped to risk food and territory competition from the other two fairy wrasses.
You
may get lucky if you add a second fairy wrasse... (If you do try, an acclimation box could significantly improve your odds.)
But chances are, the orange back won't tolerate one in a tank that size.
A better bet would be something like a leopard wrasse (if you have sand), a pink streaked wrasse, or a banded possum wrasse.