FD engagement
All said about the flip flop is correct....you will risk inhibiting the engagement of your A/THR and vertical modes which is confusing and inconvenient and even sometimes hazardous considering that this will happen during flight when you are already under a load and maybe even in a critical phase....
as for the flight director engagement, it should not be done untill the preparation is complete and all the take-off speeds have been inserted because on many occasions, when the FD is engaged prior to inserting the speeds, the managed mode for take off speeds is not considered, and you will not see your speeds on the PFD in magenta, instead you will only see the number 100 in blue, or whatever speed you have selected on the FCU also in blue.......that can only be rectified by toggling your FD off-on again.