As Linda said, our intent was not to make you feel bad, only to make sure you had thought through all of the ramifications of owning a larger bird. Especially if the birds are rescues, they are probably lucky to have a new, good, home to go to!
Even feathers that are pulled entirely grow back. This is what they do if the bird breaks a blood feather. (which can cause them to lose an ALARMING amount of blood before the feather is pulled!) They will also pull bent or damaged feathers, because new, undamaged feather will grow in faster. It's important to learn how to pull a blood feather (it requires pliers on a larger bird) so if you get one of these birds, have an experienced bird person or the vet show you how. A bird can bleed to death if left with a broken blood feather.
So if the birds have been rendered permanently unable to fly (other than the learning issue described by Linda) I believe they must be "pinioned", which means that part of the wind itself, NOT just the feathers has been removed. Again, if these birds are rescues, there's nothing that can be done after the fact, but this is a barbaric practice... much worse than docking tails and ears on a dog. It would be more akin to cutting off the dog's foot. So I HOPE that hasn't been done to these birds (though, again, I KNOW you had nothing to do with it!!!)
As far as the size bird is concerned, I think it depends COMPLETELY on the amount of space and time you have available. Even little birds should have cages big enough for serious climbing, and at LEAST hopping from branch to branch with completely open wings. Then, IMO, they should have several hours of out-time almost daily. If the birds have not been permanently de-flighted in some fashion, I would ask an experienced avian vet how best to teach them to use their wings and pectoral muscles again. It will significantly improve their overall health.
If they really will NEVER be able to fly, it puts you in the position of being even more responsible for making sure they have TONS of climbing exercise,and lots and lots of environment enriching toys and foraging opportunities.
Also remember that the bigger the bird, the more destructive they are. Plan on providing TONS of things they can take apart, shred, etc. The busier you keep them, they less noise they'll make and the less they'll destroy! (Even tiny little Sunny will eat the edges of all the papers on my desk if we don't give her lots of other things to keep her busy!!!
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It's not that you SHOULDN'T get a bird... especially if they are rescues, these birds deserve as good a home as someone can give them. But the more you know about them, where they come from, what their nature life would be like and how you can come closest to giving them what they need, the better!