Welcome to PC!!!
Personally, I would not put a bird of a different species into a cage that a bonded pair has already been living in, reguardless of species. P'lets are quite the little bullies, especially when it comes to their cages. (Ok, so Cricket and Benji are the exception here...

) But I would have a seperate cage all together. I've heard of birds biting off other bird's toes through the divider...Just a thought!