The reason you can't change the FIA's headgear is because BIS's randomization script is likely over-riding whatever you try to add. You need to wait for the randomization script to finish, then you can change the headgear. use:
null=this spawn {sleep 1; _this addHeadgear "H_Booniehat_indp";};
inside the unit's initialization. That "should" work.
*EDIT - added "null=" to prevent editor error*