Hi Sarah,
not sure what breed or age your birds are which can have a bearing on behaviour if the smaller bird is younger or a more docile / bantam breed in a pen with larger more dominant breeds such as ex bats.
If the birds are not drawing blood then the group / repeated pecking could be being insigated by a dominant hen. feather pecking is usually a sign of stress which can be caused by having too many birds in too small a space without enough to stimulate the birds. Another reason is feather eating which could be a sign of a nutritional deficiency. Though as new feathers have a strong blood supply this can also attract the attention of other hens. Feather loss in birds can be a sign of parasites so treat her with Diatom or something similar. Pay attention to her vent area and under her wings.
Giving the birds an oppertunity to get out and about if only for a couple of hours at the start of the day will really help with bullying or confinement stress but if this isnt possible then a large run with plenty of piles of leaf litter, grass or the odd stone or log that can be rolled over from time to time to reveal the bugs that hide beneath.
The bird that you describe as cheeky is what I would call "assertive"

I have a bird like this. She is my head girl and is an even tempered and egalitarian leader. Like your cheeky bird she is content to hang out with the lowest ranking bird and can be trusted to feed alongside the tiniest of new chicks. She very rarely needs to peck anyone to remain in charge but always comands respect. When she does need to remind anyone of their manners, like your girl a calm measured peck is all thats required. Your girls lowered head is a submissive and respectful gesture. Holding that position is a normal way of defusing any conflict and showing that she means no harm.
Buffy