Staying with the same amps all the way around is ideal. The next best would be to stay with the same brand as most manufacturers use the same topologies and parts through the line. If you find the sound of the Bryston a step up for your two-channel rig over the Adcom, I like your thinking as a path to an all Bryston system. Later, you could swap out the Adcom for a multi-ch Bryston or other combo of their amps.
I have never compared Adcom vs. Bryston so I can't tell you how different they will sound. If you can tell the difference between the two amps on your 2ch system then that is the change you will hear as sound pans across your 3 front channels. If you have IDENTICAL speakers (no sideways center ch etc..) and amps it will be the most seamless presentation.
Different power ratings between amps won't affect the uniformity of the volume as you turn it up. More power does not necessarily mean it will sound better. Some of the best amps made only have a few watts of power. If your speakers are rated at 90db efficiency, that means it only takes 1 watt to play 90db at 1-meter distance. The amount of power you will need depends on the type of speakers you are using and how loud you like the volume. The demands of dynamics in home theater sound tracks require the power of larger amps. If all you care about is how much your pants flap when the Death Star blows then the more watts the better. Often times the lower powered amp in the same line is more musical, because it is simpler in design and uses fewer parts. Buy with your ears and not with the spec sheet.