Re: Caique and GCC fighting
If there is aggression between the two, especially enough to cause injury, personally I would separate them immediately, at least while they aren't being directly supervised. That way, when they are together, you should be able to see exactly what is triggering the aggression, so you can address the issue directly. But if you aren't *directly* watching them, I wouldn't leave 2 birds with *any* aggressive tendencies towards each other alone, especially not alone and confined.
You can always leave their cages close to each other so that they can still see each other, and talk with each other, but without being able to directly reach each other. That should help reduce any kind of separation anxiety one or the other might have.
If you can get a better feel for what might be causing the aggression, it would be a big help in determining what steps to recommend to help rectify the situation. It would also help to get a bit more info on the birds, how old are they? How long have you had them together? Are they sharing a cage, or just spend a lot of time together?
The more variables you can fill in, the better.