Without a video of the behavior it is hard to really get a great idea of things. A vet check is always a good idea when you see something like this. What stikes me is this started around the arrival of the amazon. This could be a nervousness and fear of being alone, because the Amazon has taken attention from him. This may be a fear reaction caused by fear of the amazon and an fear of an inability to get away from the amazon.
Since you are saving up money for the vet anyway, it may be a good idea to have them in very different areas of the house with no way open from one place to another, and see if that makes a difference. I would not move the Plum Head since he is already stressed.
They may some day get along, but with the diference in sizes that is something that you would work at slowly and constantly monitor. When I socialized Sunny and Gaugan to each other, and that is much easier with a similar species, I played very close attention to both of their body language and their eye pinning. I worked at it slowly, giving them approval for tolerating. I would not even attempt them being in the same room if one was showing a repetitive behavior.
When I socialized Gaugan to Sky, a Budgie we had, there was a big size difference. I worked much the same. Gaugan never wanted to be near the budgie but she learned to tolerate her presence and never tried to hurt her. Sky was caged in a different room untill Gaugan learned to accept her. I did not want Gaugan to intimidate Sky with body manerisms in the same room untill enough time had passed that there was already some degree of aceptance since they could hear each other over time.