Thank you, everyone! I'm so grateful for all your advice and encouragement. It's almost 5 a.m. here, and all is quiet.
@camelotshadow At the rescue, there are birds from a variety of backgrounds coming in after a foster quarantine all the time. Nearly every weekend I visited, there were new birds. Opie was kept in a room with (this is an estimate) 20 budgies, a pair of cockatiels (sweet Maggie and Sam), several Quakers, a wide variety of conures, and the occasional senegal. There are currently a dozen birds (more or less) in foster homes that will be coming in to rescue soon.
So, I am not particularly worried about them sharing a room in that regard. What concerns me is the Kuiper may feel he is the only budgie left in the world who has now been placed in a room with, to him, a giant, noisy red rooster, lol!
I'm pretty sure that once I get everyone up, I will move Kuiper to the dining room. He will be away from Opie, and be able to hear and see Jelly and Juju from afar at least.
This morning, I will scan in the vet records I was provided and make an appointment for my two sweet new boys!