When answering your questions Pete noticed that there seems to be some confusion in how the ordnance phase works. Below Pete explains how the ordnance phase should be played. This applies to BFG in general and not just Tyranid players.
Reading some of the queries on the Tyranid Hive Fleet I came to the conclusion that there was a measure of confusion about the sequence of events within the ordnance phase that was worth clarifying.
Within each ordnance phase the phasing player moves his ordnance and then the non-phasing player moves his ordnance. The sequence in which the ordnance is removed is entirely up to the controlling player. This means that you could choose to move your fighters first to intercept enemy fighters prior to moving your torpedoes through the resultant gap.
This is particularly relevant for the Tyranids who move according to strict instinctive behaviour rules. You should assess instinctive behaviour conditions before you move each spore cloud or squadron of spore clouds. It is possible that early spore cloud movement could remove a condition before other spore clouds move. You cannot actually ignore an instinctive behaviour condition and sooner or later every spore cloud with a condition trigger must respond, unless something is done about the condition first.
As an example of this in the Imperial turn, a squadron of fighters is moved to within 30cm of a stationary Hiveship with 4 spore clouds on its base. When the Tyranid player moves his ordnance in the Imperial turn, all 4 spore clouds move 15cm toward the fighter. In the Tyranid turn the first spore cloud moves a further 15cm toward the fighter and both are removed. The remaining spore clouds return to the Hive Ship as it is within 30cm. The Hive Ship can then launch more spore clouds with no fear of them being immediately lured away.