It would be brave and foolish to join FlyDubai with the expectation of moving to Emirates in a few years. It has happened in the past and it might happen in the future but the rules change quickly and without warning in the Middle East. Don't read anything at all into the fact that nothing is written down about a no poaching agreement. Both airlines have the same owner, they don't need to write anything down and in any case written agreements (and employment contracts!) are flexible around here.