Free-feeding her dry food should be just fine. Read the directions on the food you're giving her. If you're supplementing the dry food with wet, then measure out the recommended amount of dry food minus a little. Try to monitor for a day or two how much she's eating. True feral cats will eat as much as they can and make themselves sick when they come in from the wild - but even our feral kittens didn't act that way when they came in becase we'd been feeding them outside since they were babies. They were fine free-feeding right off. Your Miss Hiss should be, too. But it's best to keep an eye on it.
There's a "sticky" up at the top of this forum (Feral Colonies) that you might want to take the time to read. Socialization of a feral: The Story of Lucky
. This is Sandi's cat that she referred to in her earlier post. It documents the process of socializing a truly feral cat from day one. But it has a lot of tips in it that might help you gain the trust of your "new" kitty.
I agree with MA - I think you might want to rename your kitty to something a little friendlier! I'm a big believer that names affect us....
Also, you may want to take her to the Vet. When the stool is caked on like that, it's best just to shave it off. Also, especially if she has diarrhea, it'll be easier for both you and her to keep herself clean. It can be difficult and traumatic for both you and the cat for you to try to clean off caked-on stool. Not that a trip to the vet and shaving isn't traumatic... just it's over with a lot quicker, somebody else did it, not you, and she will thank you later for it. Cats do not like being dirty, and it is obviously well past the point of her being able to clean herself. We had a kitty that had been abused, and though he skulked for a few hours after returning from the vet for a shave for the same reason, he was one happy kitty after that.
Your poor kitty doesn't feel well, doesn't smell well - and is in physical discomfort like that. Believe me, you'll be doing her a big favor.
And as to the hissing... hissing is the only way she has to communicate to you that she needs space or that's she's scared. Don't jerk or quickly move away - just slowly back off. You don't need to leave the room. I don't know under what circumstances she's hissing - is it when you go to touch her? When you get too near? Or just when you walk into the room?
Either way, reading Lucky's story will really, really help. It's worth the time. It's long, but it's wonderful, and it'll not only help you learn a lot of things that will be so helpful to you in gaining your new kitty's trust, it's just such a happy story!!!!!
I'm sure your story will have a wonderful happy ending too.
It takes a lot of time to work with special needs cats - and if your new cat has caked-on stool, she's been in a situation that has made her a special needs cat. Thank you for caring for her! Once you've earned her trust, she will be a loving, devoted friend.
The most important thing to remember is to work on her time, not yours, and it sounds to me like you've got both the desire and the patience to make this work.
BTW - all our cats are feral rescues, and though they're scared of other people and run when the door gets opened or closed - they are the most wonderful, loving, fun & playful cats anyone could ever want.
Keep us posted!!!!! You're doing a wonderful, wonderful thing.