Definetly wouldn't put those two together. I have an adult French angel right now and he is almost vicious. As far as Emperor angels, I had one in the past and it was quite the agressor as well. Unfortunately he died due to Mother Nature.
Anyway, If you were to put them together I agree to add them before they reach there adult stage and also make sure to introduce them at the same time. I would recomend at least an 8 Foot tank and lots, lots, lots of LR. They love swimming in and out of the different caves and such. Good luck. If you do decide to do it, make sure to post and let us know how it goes. I've been wanting to get another Emperor Angel for awhile but am very hesitant on putting him in with the French Angel in the 180gal I got. Trial and error if you ask me and right now I'm not ready to possibly waste a couple hundred dollars on a fish, let alone the fish itself. LOL!