Yes, any Tyranid that is not either Fearless, or Synapse (or a genestealer) must make an instinctive behavior check if it is out of synapse range.
Since the Carnifex's fearless ability is not conferred on the Guard, they would need to make a roll, and the carni would follow them.
I wouldn't worry too much about most of your creatures - with a Ld of 10 you wont fail too many rolls. I sometimes fail 1 in a game, and only once have I failed twice.