First and foremost, there's no way to predict how your current bird will react to a new bird. If things are going beautifully, I'd think very carefully before rocking that boat.
I have a quaker and a grey (a timneh), and while I adore them both, if I'd known more going into it, I wouldn't have chosen this particular combination.
The biggest concern is powder down. The dust produced by old world birds like greys can be dangerous to the respiratory system of new world birds like Quakers. I have an air purifier by the grey's cage and am fastidious about cleaning and airing out the house, but no matter what steps I take, just being around a grey is inherently risky to the Quaker.
Quakers are, as a general rule, relatively open to the idea of a mixed-species flock, which is reflected in the behavior of their wild brethren. You'll often see wild quakers mingling with other kinds of birds.
Greys are... not. In the wild, they tend to stick with other greys and shun any other company. My quaker is indifferent towards my grey, but the grey would definitely chomp him given the opportunity. I'm actually kind of lucky that she can barely fly, because he can easily avoid her just by perching someplace high up, and he has enough sense to leave her alone. (I'm hoping that some of that aggression is caused by frustration, and that it will subside as her flight skills improve and she gets more fun, healthy exercise, but I know that's probably a pipe dream).
Thankfully, about the only time they get close together is when they both want to be on me, for scritches or training, which is kind of ideal, since I'm right there to run interference. Even then, I make a point to keep space between them (e.g. one on either side of me), and I don't relax until one or the other has moved away. The rest of the time, the Quaker respects the grey's territory, and she has learned to respect his as well (early on, she tried to climb his cage and play Godzilla a few times, but that's basically stopped now).
I'm constantly aware of how much bigger she is, and how much damage she could do if she got ahold of him even once. She's learned to leave him alone because I ask her to and because I defend both territories from the other, so they don't feel the need to defend it themselves, but the aggression is by no means gone. I see how she looks at him.
They coexist relatively peacefully these days, and they definitely learn from each other's chatter, but I don't feel like they enrich each other's lives in any meaningful way. Both were single birds before they came to me, and I think they would both be perfectly content if the other vanished and they went back to being a single bird.
They remind me of a pair of neighbors who politely wave when they see each other, but never stop to chat (and one of them is secretly judgey and kind of racist). And that's actually about the best possible outcome.