Hi Chris,
Once you are through the assessment process you can state a preference and even say no to a fleet if they offer a course date (very brave/stupid) and be put back in the hold pool. The only exception to that rule is that you need a couple thousand hour jet to go direct entry onto long haul. Having said that it seems they change the new entry rules as they see fit.
From there on command hours, fast jet, rocket scientist, astronaut, don't make any difference. Everything is decided by your seniority number.
Initially you are fleet frozen for (I think) 5 years, but again if the company need you anywhere else or you want to go to another fleet you can always bid and try.
There are plenty guys and girls who like their current fleet and stay on it until a command comes around. If you're a jumbo f/o that might take while though..
The way the economy/some of its staff/volcanoes seem hell bend on braking the company it might be a somewhat hypothetical thread for the foreseeable future, but I made the jump from a jet command to BA not too long ago and have no regrets.
PS stand to be corrected by more knowledgable people
