450 gallons sounds like a lot of space and I would expect things to settle down well enough if they are both male, as long as you have enough places for the new fish to hide to get a rest from the extremely likely chasing. I would expect two solitary somewhat older dwarf angels to have each transitioned to male without another male around to suppress the transition, being protogynous hermaphrodites.
From my experience mixing dwarfs in this way, my 300 gallon has 5 dwarfs, I would expect you to be fine as long as the new guy has enough cover to lose the established fish while the established fish is doing some chasing and getting used to another male in the system. In my 180 I mixed a Multicolor angel that I thought was male with a pair of Potter's angels. This didn't go well because the 180 basically just has long linear swimming paths so the Multicolor couldn't sufficiently lose its chaser whenever one of the Potter's decided to go after it. I left them together for a number of months and last week moved the Multicolor to a separate system because it became clear that he wasn't happy - no visible fin damage but he was pacing a lot and it seemed like the pacing was because he wanted to escape (in the new system no pacing). To me it's all about the aquascape of your 450. In my 300 the rocks happen to be arranged in a way that the fish can very easily ditch a chaser and the low fish in the hierarchy, a coral beauty, doesn't seem stressed at all. A chase lasts half a second before the CB vanishes and is happily browsing rock 300 gallons away from the chaser. Will a chased flame be able to lose an aggressor?
1) Your new-comer is a little bigger. Good.
2) You have a very big tank. Good.
3) Your new addition is already very used to captivity. Good.
4) I would assume both male. Not good.
5) I'm assuming the 2.5" fish has been in the 450 a long time. Not good.
Overall, I think it will probably eventually work out well. One piece of advice I would give is to make sure that your new addition has lots of weight on it before placing it in the 450. I would feed it very heavily for a couple months. I like pellets to get extra weight on fish in addition to the usual frozen and nori. The purpose of fattening it up is buy the new fish plenty of time after introduction in case your established fish doesn't let the new guy eat for awhile...On the other hand you could try to rush introducing them to each other with the expectation that one is currently female and if you get them together quickly enough one can suppress the transition of the other, but I really don't think this is likely to be the situation that you are dealing with.
