I'm not sure i'd count on 2 quiet birds keeping a loud bird quiet. I have both types in the house. it seems the "talkers"(the more vocal birds) will get the quiet birds going. The opposite is not true. I don't think the quiet birds keep the noisier birds quiet at all. The noisy birds are always primed and any excuse will set them off, Granted none of my birds are really screamers but they talk,laugh,sing,etc alot. They do learn to mimic each other and of course the quiet bird will respond if another bird uses one of his phases. You should hear a good round of "hello" in my house, or laughing,,,OMG , my wife can get the whole flock going and it's pretty funny, it'll get anybody out of a bad mood. I don't think there's many guarantees when adding another zon to the flock.Only one way to find out.
Amazons are individuals with very different personalities. In general terms you have "talkers" and those that don't or very little.(i don't think any of my 40+ zons including wild caught breeders, hasn't spoken a human word). The talking is because it's their way of commmunicating, the birds that learn ,human speech equals attention or at least communicates their desires, ie; wellsocialized birds, will talk their fool head off. If a talking bird who is raise neglected and not well socialized ,talks ,then that's a amazon that talks. Imagine a "talker" raised with someone who goes the extra mile to interact with their bird.( like Ant and her CAG) You never know what your getting when rehoming amazons. Some are just waiting to blossom and some are just a product of their environment. Their best chance is a good "bird person" finding them.