You forget, there's flood on every installation presumably, they wouldn't need to travel from one to another
And flood combat/infection forms don't fly ships, brain forms do, as they can interact with all the controls at once, they're a meld of more than one form and the host with the information on flying the ship, so they'd have enough calcium and other things to keep the main form alive long enough (which could be years, as gravemind's been around for centuries)
I didn't mean they died in a matter of hours, i simply meant in the long term that they can't absorb other nuetrients the same way an organic creature can, so they would eventualy die unless they heavily mutated the host
http://www.brokenlinestudios.net/forums/