[poohbear]

When it comes to adding other birds to a cage with a bird that has established it's territory there can always be a problem.However I would have thought the ideal in your case would be a Bengalese Finch (Society)
They are the most inoffensive bird on the planet and get on with their own kind and others with next to no aggravation.
The ideal would be to put the birds together in a new cage so there is a level playing field, both being in a new enviroment.If not then moving the position of the perches,feed pots etc. can help.The established bird then has to adjust the same as the newcomer,and each will pick it's own small territory in the cage.
I always do one or the other when introducing cock and hen finches for the first time...by the time the birds have adjusted to their new home they have more or less accepted each other.
Paul.