Introducing adult cats

I have had my 1 year old female Dori cat since a kitten and recently decided to adopt a cat who had been abandoned and brought into my workplace (I work at a vets). Three days ago I brought our new cat Garfield home. He is a nine month old neutered male (dori is also neutered). Dori disliked him from the start and growled at the sight of him. We took this as normal and gave them space and quiet. All seemed to be going well on the first day, she was still hissing and growling at him but he seemed completely relaxed in her company and she was gradually starting to relax to. (Both cats have previously been happy and playful in the company of other cats Garfield grew up with his brother and Dori plays with all of the other cats in the area who regulary come into the house). By the evening we were happy with the progress until out of the blue Garfield attacked Dori!! He chased her all over the house and onto the windowsill in our bedroom where she has been ever since. The one time she ventured downstairs he aggressively chased her onto the windowsill again. He is an exceptionally large cat and she is very small. After asking advice from work colleagues and looking on the internet we have started from square one and put garfield in the spare bedroom with plenty of food / water / toys / litter tray. What we are planning to do is gradually get them used to each others sent by putting things they lie on or things we have rubbed them with near each others feeding place to try and get them to associate each others scent with something positive and letting them take turns in roaming the house while the other is not present to get them used to the scent. The problem now is even with garfield safely shut in the other room Dori still wont budge from the windowsill. She gets extremely upset and aggressive if we try and move her as she is terrified of coming face to face with garfield again! Yesterday we fed her and put water on the windowsill as we were concerned for her health but we have not managed to persuade her to use her litter tray for at least 48 hours now. We have decided to give her some space and let her do things in her own time and hope when we return from work today she will be roaming the house again. Has anybody else been through a similar situation?? And what did you do to overcome it??? Any advice would be much apreciated as the last thing we want to do is make dori unhappy but it would be heart breaking to have to re home garfield as he has had little stabilty in his short life. So if anybody knows anything else we can try that would be reaaly helpful.