I don't think I would "poo poo" just yet on the Quaker.  It is possible that he/she talks and is tame.  However, it just sounds like the Quaker (which are rather territorial critters-of a cage, toy, person, another bird, etc...) doesnt want you near the SunConure, and is being protective of him.  So he's going after you to keep you away from his buddy.  Acceptable?  Of course not!  How long have you had the Quaker?  Was he shipped to you or did you go to the breeders house and watch this bird and how the breeder was handling him?  Also how old is your Quaker? 

Don't read more into it with the other two birdies you have)  They still are tame im sure, but sometimes when a new bird comes into the flock things just change.  It might take them time to "go back to normal".  Theres also a possibility that this little Quaker might not fit into your flock, but as irritating as he might be to you, if you cant get whomever had him to take him back (if this is the route you choose to take) then its your responsibility to put him into a good home and not dump him into the first lap that will take him. 

Stacy Shue
Blake Aviary
www.blakeaviary.com