Its a hard one, as Kaks are not the easiest to pair with another species. I personally would leave her be, as she is still at the taming stage. She has you, and a lovely cage, so at the moment is it necessary? If you put another female Kak, it should be okay, Maybe Sal would say otherwise. If you put a male in, she will lay eggs of course, but they bond for life, and its possibly the best partner for her. Then would the male Kak be okay with your new rosella out of the cage?, i really dont know. I have a Kak, that was raised with a budgie of mine, and they were best friends for a long time. Then Lennie my Kak, got a bit rough, and My budgie Graham cannot fly, so he cannot get away quick enough. Lennie is in the avairy now, with budgies, but they are not bothered by him, and can get away when they need to. I was told no to tiels, as they are too soft. There is a good Kak forum, and they may have some ideas for you. Are they fighting? www.kakariki.net
When I had my kaks they were in the aviary with budgies without problems. Don't think I'd bother with inside though - as long as they have toys to play with when you're not there they don't need another bird. Plus if you are lucky enough to tame the rosella they could be companions.
i agree with karen and scully, they are pretty persistant breeders even if you dont want them to breed so a male wouldnt really be the best idea for you. A female would be ok as long as they are both young enough, but once a kak reaches maturity (which is about 8 months) another female being introduced could be seen as a threat and not accepted.
with enough to do in the cage, and enough love and attention from you she should be a very happy single birdie you could always test the water and have the two out together and see if they would accept being flight buddies? my Kiwi wont tolerate having the budgies in the same cage as him (he's too old and set in his ways to let them in) but he enjoys having flight time with them and they share the toys lovely! i hang little treats all around the room to keep them amused so they actually do some flying while they are out instead of just perching on the curtains and kiwi will even take some to sherby (my 11 year old budgie) cos he knows he hasnt got the flight left him to get them himself lol its very funny to watch!!
i would leave her be for now and just see how she goes, you could end up with a VERY cuddly kiwi if she's happy with you as a partner if she starts pining for company (eg sits in corner quietly and refuses to play with toys that you know are favorites) then have another think about it then - or - as i said, if she is young enough try and find another young female
Thanks very much for all your advise - I've had a whole 2 days in the house with both the birds and Kiwi and Joey are showing signs of inter-acting with each other so we're going to see if we can introduce them both to fly time together and see how it goes. Much thanks again for all your advise
Lol I'll put some more pics up in the picture section
They are getting along better and better with each fly time together and they chatter to each other all the time as their cages are right next to each other. Kiwi is always in and out of Joey's cage scragging whatever she can out of there