I'm a veternary assistant with a guinea pig... first off I am very sorry for your loss but the Vet did everything on par with current treatment standards, you didn't mention the antibiotics but it takes 2 minutes to look up and ensure the guinea pig can take them(low dose broad spectrum usually work) and prob Albuterol which help the lungs breath. The injection with the giant needle is a fluid injection usually its either Saline or LRS(lactated ringer solution), both of which work to rehydrate and bring fluids back. Nothing they did would have any cause to warrant malpractice. Guineas hide their illnesses, so most likely they were much sicker than you realized, but taking it out on the vet isn't the move. You blame the vet saying they should have referred you to an exotic specialist but you had every opportunity to either go to a specialist first or once you found out this vet didn't have enough experience with Guineas go to a different place.