I have a coral banded and a cleaner in my 75. The cleaner is about 3/4 of the size of the coral banded and they're doing fine. The coral banded is a lot more shy than the cleaner and tends to come out more at night (but in pairs they're supposed to be more of outverts!). The coral banded occasionally will have a short chase after the cleaner, since they're usually close together, but the cleaner can easily get away in my 75. By the way, I love watching both of them!
So if your coral banded are smaller in your 120, I would think you would also be fine. Just keep a close eye!