Will rehomed birds teach each other bad habits?

[Lake Desire]

Hello! I was wondering adopting a rehomed parrot from a rescue or Craigslist so that my older nanday conure, Jade, could have a friend. I posted about Jade a few weeks ago: she became a feather plucker when I went to college, and generally has anxiety due to a cat attack ten years ago. She came back to live with me and is overall much happier, but I am still thinking about adopting a friend for her so she can feel like she is part of a flock. I have a few questions:

Would Jade teach a new bird to feather pluck?

How important is it that the birds are the same size, if I am not planning on having them out together unsupervised? (I am worried about Jade plucking a new bird's feathers.) I have been emailing with an owner of a 12 year old sun conure who is in too small of a cage and seems to be regularly rehomed due to kids getting bored with her. My heart wants to take this bird in and give her a loving home! However, I have also talking to a couple who needs to rehome their yellow-crown Amazon due to aging (their aging--the bird is 7). They've been having trouble finding reliable people who are committed to the bird long term.

Anyhow, any advice on introducing a scrappy old bird to a new flock mate would be appreciated!
Thank you!

Edited to add: Just to be clear, I am not planning on housing them in the same cage, I just want to keep them in the same room after quarantine is over.

[Polarn]

Even if not caged together if "agressive" behavior is shown by one towards the other having one bird losoe in the same room as the other one is caged may even end bad since a bird walking or climbing another birds cage can still loose toes etcetera. as for introducing mines I consider myself lucky.. however I recently moved my budgie and tiel to another room because the tiel started to try interact with my amazon and I dont want them to interact due to the sizedifference (I might try get em into the same room again once the hormones are over) but she would basically chase my amazon around and he would fly off, and she was a persistent bugger so after a while when he kept ignoring her she would start pulling his tail... so I chose to separate them since I 1. doesnt want the budgie to find it reinforcing pulling another birds tail because it gives her attention and 2. I do not want Polarn to bite or something causing biting to be reinforced by the fact he got left alone... So for now the little ones live in one room and the big ones in another....

Anyways as to teaching each other behavior... well yes and no If you for example have a quiet bird that gets tons of attention and you adopt a screamer, his screaming might actually decrease because your ignoring the screams but reinforces the quiet, this also means you have to reinforce the silence if the screamer goes silent. as for plucking, I dont really know weather they would teach each other that since plucking is a selfmutilating/destructive behavior... and I have a hard time seeing the reinforcement here (unless you start talkign to the bird telling him not to pluck himself etc every time you see it, then your the stimuli (atleast one of them) for the continued behavior).
Neither of the two birds you mentioned are to my knowledge known to be silent birds though... Most people (including my 10y old nephew who talks nonstop, and I consider him louder than my zon) wonder how I can live with the noise, but as of right now I havn't noticed a noise in quite some time sitting here but I know if I were to replay the audio in here I could probably hear him making noises every now and then... but there is a difference between a screamer and bird presenting their natural behaviors (as in my amazons greeting the sun ritual, that is quite loud to be honest). a sunconure I've never lived with (don't even know what their called in swedish to be honest) so I cant tell you if their "noise" is just that its frequent or if its both loud and frequent or if its just loud... I have a hard time seeing them being loud though (couse their so small) but then I'm used to my zon and RB2. They may just as well be as loud when screaming as these two though, I dont know.

Now to good habbits, My zon just the other day thought my Galah how to play with toys, he would play for a bit then back off and wait for the Galah to make an attempt then she would back off and he would do it again, this repeated itself for quite some time.. Today is the first time I've actually seen my Galah climb on the toy ringing the bell etc.

But yeah they pick up each others sounds and they may reinforce each others by responding equally loud. My zon sounds like a budgie every now and then wich is just cute to be honest... and I encourage him todo that sound couse it is the least loud sound he ever does for attention, well that and saying "wooap wooap".

[Lake Desire]

Thanks for the thoughtful and quick reply! My Jade is a squawker, but doesn't scream unless she sees the vacuum or the mop. (I don't cover her cage, but may need to start covering it when I mop!) The amazon we are considering doesn't screech or scream, but I suppose Jade could teach her to. I assume any conure we adopt would exchange many squawks with Jade! I don't mind squawking at all--birds are just being birds--but I certainly don't know that I want panicked screaming birds all day.

I was not planning on letting either bird sit on top of each other's cage or be out unsupervised... that just doesn't seem fair to the bird inside the cage to have a big meaning come sit on top! (Our poor Jade once had our growing baby chicks perch on top of her cage when they escaped their cardboard box, and little Jade was amused but indignant!)

[Polarn]

about the zon not screaming, more than one person have adopted a silent bird and ended up with one that makes sounds, not nessessarily a screamer but one who starts vocalizing. My zon took about a month before I heard him the first time (part from small little noises) now he is dancing around greeting the sun at the window every morning but yeah birds are birds and they have their vocalization and I've just let em have it but I've never had one screaming nonstop constantly either.

[pennyandrocky]

no they don't learn plucking from other birds it's either mental stress or health problems that cause plucking.my is a plucker and my has never plucked i don't imagine that plucking feels good either it's like people who have suffered trauma in life and cut themselves. mine actually flinches when she does it.there's also a huge size difference between my 2 but mya is becoming more tolerant of penny i can have both on me at the same time and mya is even willing to share her cage with him now allowing him to grab a snack or drink from her water and they hang out on top together too.