Hi Rachael, how old is your guinea? I agree with your thoughts on what the problem is, it does sound like lice.
Whichever shampoo you choose to treat with, it will need to soak in for a certain amount of time at each of the weekly baths. I tend to give a thorough wash and rinse before lathering up the pig a second time and letting it soak in.
FleaorDie is usually left on for 10 minutes and then rinsed, while Lice n Easy (from
www.gorgeousguineas.com) is left on for 5 minutes then rinsed. I have no experience of using other anti-parasitic shampoos, although I know there are others.