If mechs are allowed, surely most other vehicles are too, including small space ships. Once you start incorporating large-scale weaponry like those on a space ship, you're gonna get to high tier really quickly though. It seems any vessel worthy of having three crew members will pass tier 7 quite easily if we don't hold back. That is, if we're going with a scifi setting, which i assume. So that might be a problem.

From a writing standpoint, i dunno. As stated today in the other thread, fights tend to get chaotic when more than four people are involved. Now imagine four people trying to work together ON THE SAME POST to defeat a fifth. Seems cumbersome, but it might work, maybe.