I had a Burmese boy before I retired from vet nursing.
I then brought home a female (desexed) DSH (Annie-Anne) who had been dumped at our surgery and the 2 were great mates.......................however, any cat/kitten that came into our household after Annie-Anne, she absolutely hated!
When I brought my 2 Siamese kittens home, she used to bash them up but being intelligent creatures, they soon learnt to scream their lungs out if Annie-Anne so much as looked at them so I'd come running and save them.
I'd get there and tell them that she was nowhere near them and of course they screamed at me (as only a Siamese can) that "had you not come along, she was going to kill us!"
The threats of violence gradually stopped but then Annie-Anne let us know that she was not happy Jan (to say the least).
She sprayed everywhere. Curtains, furniture and her favourite on the bench tops was canisters, the kettle, the fruit bowl and the toaster.
Thank heavens the toaster was never switched on when she decided to combine electricity with a connecting stream of pee.
We sought vet treatment and Annie-Anne was prescribed hormones.
We also tried remedy rescue from a naturopath.
I don't think Feliway was on the market back then.
End result was that it was obviously stress and I refused to drug her up.
Vet advice was that the stress had to be removed. Either Annie-Anne or the Siamese must go.
I chose to do neither.
I took down the curtains and removed everything from the kitchen bench so that if she sprayed on the bench, I only had to clean it up and disinfect it.
Everything on the benches were put into cupboards.
I tried to keep her in a separate room away from the Siamese but it only increased her stress even more and wasn't really fair on her as she was there first.
We coped ( even though our visitors refused to eay fruit or toast).
The spraying did decrease but never completely stopped and our Annie-Anne left for the bridge aged 16.
I guess my experience isn't what you want to hear. We just loved her so much that if we couldn't find a remedy, then we would arrange our house to suit.
There are other things that have been introduced to the market now so maybe you'll have more success than we did.
All my very best wishes and will you please post about how it goes?