I understand the concern about foxes and fighting and other cats etc.
In my experience, it takes a bit of time for a cat to develop their boundaries and make sure everyone in the neighbourhood - including foxes - knows the score. There will be months of negotiations, parlaying, treaties, testing the boundaries and so on. Then it settles down.
For me and my cats it’s never ended in disaster, male or female, large or small. One of the fiercest cats I ever lived with was a tiny little bitch-cat-from-hell who was easily the match of any tom dick or harry in the neighbourhood, foxes included.
And I've had to deal with wounds and the abscesses that can arise from wounds (cat spit can be right toxic). The Auld Warrior was in an ongoing war with an equally fierce opponent for almost a year before they finally made a truce that worked for them both. He was to the vet so often that they waived the consultation fee for him, so long as it was a bite wound. When the war was over, it was pax aeturna.
When Cat was little, he was bullied by the local Big Boys. I was worried he’d never be able to stand up to them (he’s a gentle soul). Then one day, he did, and after three months of scrapping and quarrelling and fighting and swearing matches, they stopped bothering him and they now don’t even come through the garden. Cat prevailed!
Depending on the cat and the circumstances, I’ll sometimes stare down the opposing cat, sometimes hiss at them, sometimes just observe and not interfere. I don’t know if I’m doing the right thing or not tbh.
HOWEVER…!
This is anecdotal, and I’d never suggest it as policy for anyone else. It’s just the way things are for me.