Hi tricky, I have a little experience in this area so I hope I can give you a few ideas.
Unfortunately you can't really do much about the bullying, hens can be really nasty and I have spent a fortune at the vets getting a chicken better after it was bullied so much it lost an eye. She is absolutely fine now, better than fine, but my guilt led me to spend over 5 times her cost in putting her right! And she gets on great with all the other hens now, in fact I can introduce new hens and chicks and re-introduce my broody with her chicks with very few confrontations. I must admit I didn't see the need for a cockerel at first but I have one now for breeding and even though he's a bit of an old woman at times he does seem to keep the girls in check! That might be worth considering, 5 hens isn't too bad for one boy.
Otherwise you could try more feeding/drinking stations so everyone has plenty of choice but make sure your little hen is still able to eat enough before getting pushed aside, which is kind of inevitable.
Or try taking the little one away from the others, but only to feed, if she is separated for any length of time you have all the problems of re-introducing her to the flock and establishing the pecking order all over again.
If you do keep her apart to eat I would add some nice things to her food to boost her back up, when I want to spoil my lot they have warm porridge or weetabix with live yoghurt on top, and leftover pasta esp spaghetti which they think is worms lol!
I also put cider vinegar, garlic, poultry spice, citrus extract, nutri drops in their food or water as and when - get a stockpile of remedies in, I got it all from happy chicks but I've seen a lot of these in my local animal feed/country stores.
While you're at it get some of the violet spray, if chickens see red they peck it more, so if you spray it purple it sort of disguises it, plus it has some anti bacterial properties as well I think. I prefer to have everything to hand after getting caught out last year, and I used this recently when one of a brood of 10 chicks was getting picked on. the evil little siblings were hanging onto her comb, it got very red and swollen so I made it purple, it did the trick!
good luck with your hens, you'll soon have the hang of it!
Best wishes,
Helen