well I'm gonna beg to differ on this one, and say that a majestic should be ok in a 125 or quite a while. Just make sure he is at least as big as flame, preferably bigger. The two tangs will kick up stink for 24hrs, but the flame is the one to watch....... his aggression will take at least a week to subside - if the majestic is bigger, then the flame will just display and the majest will just ignore it...... but if they are equaly sized or if the majestic is smaller, then the majestic could well be killed within a week
As far as the tank size....... it will take acouple of years to reach a size where it begins to outgrow a 125..... a 125 is 72"x18"x24" right? effective aquascaping and good water flow is the key here - and so long as you don't add anymore big boddied fish (as in wrasse, tangs, b'flies, angels etc.) then you should be ok. If the majestic starts nudging up to around 7 or 8 inches, then it is time for either a new tank, or a trade in...........
should be reef safe with all but the most tasty LPS etc.
HTH
Matt