Cats who lived without any sort of boundaries...are bound to resist your house rules. Perhaps this is just the nature of cats who have not been domesticated pets for as long as dogs. Of course, this could also be a nature versus nuture situation. 

Fights for territory reach across many species of animals, including those in the wild, and also, including people. Cats are not exempt from fighting for territory and feral cats brought into an enclosed space may exhibit several territorial behaviors including: 

-marking by going outside the litter box

-marking by clawing or otherwise damaging items that smell like other animals

-picking fights with pets already in the home

But don't worry, there are many things you can do to help ease these territorial disputes between your feline friends. One of the things that will help you determine how to go about solving these problems is being a detailed Observer within the first few months of introducing your feral cat to your home. 

If there are litter box issues, one of the ways to solve that is by getting more litter boxes. This is an instance where you want to follow the rule of one litter box per cat plus an extra. Another thing about litter boxes is that they need to not all be in the same contain area.

If any of your cats have claimed a specific area as their territory, of course, having multiple litter boxes contains to that area will make that a hostile territory for your other cats, which will lead to them not using litter boxes within the territory of other cats. 

This same logic May apply to any scratching post oh, cat beds, cat houses, cat perches, food bowls, watering bowls, or cat toys. Despite the fact that we as humans have been brainwashed into believing that sharing is caring, cats really do not believe this same thing, especially not when you're more established cat feel that a newcomer is encroaching on their territory.

Yes, cats can feel jealousy. It is another way for them to claim territory... Over their caretaker, also known as you, the cat parent. As with any other territory battle, your cats may be unwilling to share, especially a cat that has not been socialized to live with other animals, whether that be a feral cat or a cat that has previously been a solo pet in the home.

This is not to say that a feral cat (that was not a part of a pack) is incapable of getting along with other creatures, but that it will take some adjustments on your part to make it happen. It is also more realistic to expect that some pets may never truly like each other or find a way to get along. But there are many things to be tried before you give up by creating a happy-ish home with your adopted cat.

Some things you can do to help put your jealous cat(s) at ease include showing affection to each of your cats in view of each other, increasing bonding time with your cats, i.e. cuddling, watching movies, brushing, playing together, and also enticing your cats to spend time together under your supervision.