Why didn't I think of turning our discussion into a meme? Because you're the awesome one, that's why.
Anyway, here goes...
Hamish: Probably the same animal he'd have as a patronus: a ram. Why? First, because sheep. And secondly, because they're calm, placid creatures (shut up, Cass) but have no qualms about attacking you with their horns if threatened. Also, their fleece is as soft and curly as Hamish's hair.
Samir: A hyperactive kitten! Either a Sphinx or a Siamese because of their big blue eyes and the fact both breeds descent from the cats that were worshipped in Ancient Egypt. Alison might have a different idea from mine, but I see Barney as a puppy person and Samir as a kitten person, both adorable and running around like crazy all over the place, but with different traits according to their species.
Zoe: Also a cat, but something a bit more dangerous and predatory. A black panther, perhaps. Like all felines, she likes to stalk on her prey and play with it for a loooong time before eating.
Tiago: Part of me wants to say a sloth because he's so placid and easygoing, but that would be the pot talking. :P However, because he's playful and loves the ocean so much, the only possible choice for him would have to be a dolphin.
Cormac: This one is the trickiest. I want to say a horse, but that's the animal that symbolizes his family (particularly his brother) and why he's trying to detach himself from their values and beliefs. Whatever it is, it's got to be something big. I'll think about him some more - and in the meantime, if you have any suggestions feel free to drop them here. :)