I wouldn't use your dog shampoo for guinea pigs; that could cause problems!
Ideally, you bathe your boys at the same time with the same shampoo. if their bum end is very dirty, you can also give them a quick "bum bath" instead of a full blown one.
Here are our tips for bathing, including ears and frequency:
http://www.theguineapigforum.co.uk/...-Guinea-Pigs-including-Ears-amp-Grease-Glands
The best shampoos are from gorgeous guineas, which are specifically formulated for guinea pig skin and don't contain any potentially harmful ingredients. Skin parasites are species specific, that is why guinea pigs don't have flies (and why any dog flies can't survive on them). Mites also don't transmit and need the appropriate ivermectin interval treatment after a proper diagnosis from a vet. Please do not home treat with shop products; you can make things worse instead of better, as those products are too low dosed to cure.
http://www.guinealynx.info/parasites.html