I don't think chickens will have any more diseases than any other domesticated bird or pet for that matter. There is the bird flu but there is no reason for your bird to catch it especially if it's in the house. The roos do crow, usually much less loudly than a larger bird, but individual birds can be manic about crowing, or mostly silent, you never know?
Good luck with your choices!!! I just have eggs in the incubator, no Seramas yet, but I'm so excited and am hoping the two that got this far, and are due to hatch early next week make it!!!
Chickens can carry a lot more diseases and seem to get them more often than some other birds. I'm not talking about bird flu, I'm talking about Mareks, coccidiosis, fowl pox, mites, lice, infectious bronchitis, coryza, Newcastle, Aspergillosis, etc. etc. They seem much more susceptible than other barnyard birds like ducks and geese and some of these diseases they can carry without any apparent signs.
I'm not saying don't get them, I'm just saying be cautious. It would be awful to infect a beloved pet that you already have. Personally, I would never have them around my Sun Conure. I might be overcautious, but I'd rather not risk it.