One tube per animal. Back of the neck is ideal. I try to avoid washing guinea pigs (and never wash rabbits). The only pig I'm washing at the moment is Winifred and only because she is 7yrs+ and can't maintain a clean underside. Prior to that I only washed her maybe once a year and only if required. The vet has recommended Aloveen pet shampoo but I have used previously shampoos from Gorgeous Guineas (ordered online from the UK) that were really good. I just get the sample sizes because as above, prior to this stage of life for Win, I only wash when absolutely necessary.
If you do give them a wash:
* Make sure where you are bathing them is suitable. Where possible use a tub on the bathroom floor. Some guinea pigs just about fly out of the water because it is scary. Have someone help you if possible. It can be a challenge to keep a wet, slippery, terrified pig safe from injury.
* Take care with the water depth and temp. Test with you elbow and make sure they can stand up easily.
* Avoid getting their face, ears, eyes wet. Wash neck down only.
* Rinse carefully. Again, be careful of water temp.
* Dry quickly. Microfibre towels are wonderful. Not all guinea pigs will tolerate a hair dryer. When using a hair dryer keep your hand on the pig to check temperature. I find I have to hold the dryer nearly arms length away to avoid it getting too hot on their fragile skin.
* Minimise stress before, during, afterwards. Some pigs really don't like a bath and become very stressed which is why I avoid it. Make sure after drying them thoroughly, you keep the temperature stable in their room. Nice and quiet, easy access to food and water.
I strongly recommend to have a vet look them over and help narrow down on the cause of the itching though. The will be able to tell you precisely what is going on.
If you're a Sydneysider, SERV are excellent. They will test the hair during the appointment and let you know if parasites are present.