Just keep an eye on them for a while before leaving them alone...
I have a YCA, a rosella just like yours and a tiel.The two last ones ignore eachother as the other doesn't even exist for them...

(very polite...

)
I tried a couple of times to put them together so they always have company but none of them wants that, they will go for each other both usually when they are sitting on their cages and I leave the room, I always had to run back hearing the screaming to separate them so I put their cages far enough(they are both not fully flighted).
I had the idea once to put them in the same cage(that was in the begining), God knows how the tiel came out with some feathers left on (

), but although she is much smaller than the rosella , also quite small for a tiel, she will not loose a chance going for the other as well...
My rosella is also very teritorial, won't allow other birds in her cage, I once gave her a toy which also had a mirror, she became so agressive she nearly broke it to pieces,attaking it so severely .... I thought what if she will do so with my tiel, she will have no chance....So for my birds, better they stay far for each other....
Take extra care, keep us updated!

Good luck!
