Hello!
I'd have slightly different advice...
Does the cat flap mean you'll always keep her in at night? This is a good policy firstly because cats mostly hunt our other wildlife at night, so keeping them in is good for the birds and small mammals, also night-time is when most of the territorial fighting takes place so keeping her in will help her avoid potential border disputes.
Unfortunately, you're unlikely to be able to watch every second of her life for the next 10-20 years so I think you have to bite the bullet a little and let her go; you have to remember that even a young cat is armed with all kind ofhandy senses and behaviours totally foreign to us; if she strays into another cat's territory she'll know it from the scent marks left everywhere, she'll also know if the resident cat is male or female, young or old and have at least a fair idea of how big and tough it is as well, so she can't wander blindly into a risk, it will always be calculated on her part as to how far she wants to push it and she'll likely be very cautious.
Cat fights themselves are also more noise and bluff than anything; all predators, especially solitary cats, a hard wired to avoid confrontation and fighting whenever possible because a serious injury stops you from hunting, in the wild that means you starve to death; that's a lot less relevant to domestic cats but instinct is instinct and it's been there for over 50 million years, this is why most cat fights you hear build up for ages with the cats circling each other growling, most times one will back down and just run off home because it's obvious who the winner will be; an 8 month old kitten is always going to back down and run for home like crazy rather than stand and fight, so it's unlikely she'd get into much trouble based on this alone. When you hear cats fighting it's always two very closely matched individuals that have sized each other up for as much as 20 minutes or even for weeks beforehand before going at each other and it's usually settled very quickly as soon as one gets the upperhand.
Without a doubt, going out cats face a riskier lifestyle than indoors cats and are bound to get in a few scraps in their lifetime, but there is a whole social and moral system of rules there in the way that all cats behave that protect the individual straying into the wrong place at the wrong time, so I think if you want her to be outdoors just let her go for it and leave her to learn and get "streetwise".
I would be more concerned about cars and consider taking her for a walk near to a road and let her get a good scare from a passing car, it's common sense for cats to be scared of them but if she associates road tarmac with big noisy smelly scary thing for a while it can only be a good thing.
Good luck!
Steve
