I have a maine coon neutered female(Bindi - yes I know not politically correct but not my choice and who has been a queen) 3 years od and a semi feral moggie (White Cat - yes I know sorry for name) whom we think is female and must have been neutered at some time (not by us).

White cat has been with us 4-5 years and lives outside or comes in but only when it suits her. When I got Bindi in May there was a lot of hissing and growling and paw swatting. I couldn't separate them in the traditional way but I did do a lot of transference smell stroking and talking to each of them in turn.

We now have a cuddle up to each other situation and a lot of sniffing bottoms which I find intriguing and confusing - why? It's like dogs!

The neutered male is now definitely number 3 in the pecking order even though he is twice their size!

So it does get better over time though I would definitely advocate supervision in first weeks.