Well, Pilot would definitely be Scarecrow and Engie would undoubtedly be the Cowardly Lion (he even has the ruff!), so I would say Dorothy. Captein could be Tin Man since they always carry the heart mug around.
I'd say Dorothy, since I've read the actual book. Being dragged into situations over which he has no real control over and with only half an idea of what's going on and just "following the yellow brick road" AKA the Captain's crazy ideas. Only Dorothy never really questions whatever she did to deserve it
Snippy is Dorthy. Pilot is the Scarecrow, looking for a brain. Engie is either the Cowardly Lion because of his cowardice, or he's the Tin Man looking for a heart. Captain is the Wizard, but more awesome. He's also Glinda.
snippy would be tin man, no heart, kinda cynical. captain would be the wizard. that goes without saying. (or dorothy because, well, "we are off to see zee wizard~!") pilot would be scarecrow, because who needs brians when your pilot? engie would be the cowardly lion, dont ask it just fits in my mind. the red alien stuff would be wicked witch green alien stuff would be the good witch (glenda?)
Sniper is the Cowardly Lion[reluctant and... well, cowardly], Engineer is Tin-man, Pilot is the Scarecrow [a 'logical' solution to everything=you are all shoes.], Captain is Dorothy [his mug is Toto] Biomass is the Wicked Witch. ANNIE is the Wizard. Lifealope is the Good Witch.