Rabbits you need to rule out if it's an unwanted 'visitor' like mites /lice/ringworm.
Mites tens to be on the rump area and presents in a V or Y shaped hair loss with snapped off hairs.
Lice is more like a dandruffy dander.
You could try Lice 'n' Easy from Gorgeous Guineas. They do a Melt as well that you dissolve and apply like a conditioner.
Their website is good,.They do some £2 sample pots, you could buy a selection.
I bought Lice'n'Easy, Just for Boars and Maluka and Neem one plus a Calenda Cream (healing)
If it's ringworm, you'd be best to see the vet, it's a nightmare to shift.
Lots of the GG shampoos are for GPs over 12 weeks. They do have some some mini-pigs but check on their site .
I'd have thought they get enough from their diet WRT to oils .Difficult to add without giving them the runs and fatty foods like avacado is a no-no for guineas.
What are you bedding them on? Might be worth changing the type of bedding?