I agree the size difference is potentially a significant issue and adds complexity and risk that must be managed.
This is somewhat hopeful on the surface -- it'd be more worrisome if the bigger bird was overly bonded (leading to aggression), but it sounds like you've got some healthy independence in your Amazon. That doesn't remove the risk of mixed sizes, but might make it more manageable.
Conures don't appear to know they're small, or care, and this created some very scary situations until mine learned to keep distance from my Severe.
How long has your Amazon been an only bird?
My Severe was an only bird for 10 years (the majority of his life), and his skills for living in a flock aren't developed. I'll add that he was re-homed because he chased the previous family's grandkids (something that I didn't think would be an issue in my child-free home, but he still exhibits this behaviour toward my other birds -- large and small).
Conversely, I adopted a Military who had other macaws as flock mates and he is skillful at navigating flock dynamics.
My experience with these varied sizes coexisting (not harmoniously) is that it comes down to how aggressive or passive each member is. None of mine interact directly, and that would be fine and easy if that was the end of it. My Severe complicates the mix because his aggression causes him to sometimes chase the other two (increasing difficulty). My Conure and Military respond by flying away from the Severe (reducing difficulty). With this, it's easy to see how it'd be more far more challenging to manage if multiple birds were aggressive, or if some didn't deescalate.
Over time you can learn what triggers aggression to help manage the dynamic, but some things can't be avoided (like hormones amping things up). It's definitely not an easy road, but I've been able to navigate my individual personalities to find a living situation that works for everyone. I am sometimes envious of people with single species flocks though