Aww she's gorgeous
I think I would get her checked out at the vets. It looks in the right area to be the centre of a typical crest, but it looks too big - crest/rosette centres are usually only a few millimetres wide at most.
Does the skin look normal in the bald patch--or does it look redder, scabby, scaly, flakey or spotty?
There are other forms of fungal, not just ringworm.
I would get her vet checked & sorted before adding her to the rest of your guineas as a precaution, it will save you a lot of time, money and stress if it turns out to be fungal & you've quarantined her
I have a long haired, abyssinian mix with only a few rosettes in her hair, where she's so long haired they don't look like typical rosettes/crest but her hair sticks up in clumps, the hair going off in all directions (with a cute comb-over look on her head)!
I'm not sure of the consumer laws in the States, but in the UK if you've bought an ill animal that needs veterinary treatment, the pet shop where purchased is required to pay for treatment if it's within a short time (allow for an incubation period of a fungal outbreak).
As impersonal & unsensitive as it sounds, in the UK it's basically purchasing damaged stock and they are required to refund, repair or replace!
I think I read of a US member that had to surrender her pig back to the store for treatment, but claimed their guinea back afterwards (although personally I'd be apprehensive to do it this way & worry I didn't get her back). In the UK submitting vet receipts is usually the way it's done.
Good luck & let us know how your pretty lady gets on
Xx