It feels like we'd be going in separate groups of two. Cormag and Iscariot would be taking a military airship (hopefully my reasoning for it will be plot-justified in a way Nova will accept), Luna and Caramon would be swimming (?), and Marti and Ludus... well, they'll either get a boat or pull out some Ride Chasers. The latter seems more appropriate, considering Marti. If you're good at math, you'll notice that we have 7 Biometals, which leaves an odd character out: Model W H [I'm doing this as a Megaman nerd to not confuse my future self.], who would be justified in this case because extended flight would probably only sustain one person anyhow. That is, if the player comes back; might be worth
Speaking of [the real] Model W...
I have an idea: Why not implement a team of Maverick Biomatches? Either through evil dupicates of our current Biometals, copies of a big bad Biometal with different elements- whatever the case, it might trickle in excess players if there was a prospect of not just being able to play as a bad guy, but a UNIQUE bad guy.
I dunno where you're going with the story though, so I can't say for sure if this is acceptable with what you're going for. It's fine either way, though, as seven players for the Biometals should be fine also.