I got Cait when she was 16 weeks old and she had the same response to a slicker brush that your Fiona has. I think she thought the brush was nipping her. I got a soft brush for her and used it for short periods until she stopped jumping every time the brush was brought near. The soft brush unfortunately didn't do much to her thick fur. It also had a pin brush side and after Cait accepted the soft brush, I'd gently and briefly use the pin side on her, stopping if I could before Cait started flinching or playing. Helping with training Cait were my two older dogs (husky mixes) who love being brushed. I'd brush them with Cait watching. I'd also pet Cait after and between using the brush, telling her how I was going to make her beautiful and that she was a star. Cait is a sucker for this kind of talk.
After a few weeks of her accepting the soft brush, which she finally did happily, I reintroduced the slicker brush. I'd use it briefly, stopping whenever she showed signs of restlessness or tried to bite the brush. Soon, she accepted it longer, fortunately in time for when she started blowing her puppy coat. Now I pick up the brush and say, come and I'll make you beautiful and she comes right to me. I still have to stop, usually when I've only half a sheltie brushed but one of these days, I expect she'll be standing in line for her turn to be brushed with the other dogs.
Good luck. Some shelties, like Cait, can be awfully stubborn when they make up their mind about something and it isn't easy to get them to change their minds. But it can be done. Still, Cait did manage to find the slicker brush and chew off the end of it. Never mind. It still works.