Hi Tino,
Welcome
In my opinion the GY401 is best paired with the 9254 (I have 3 600 class trexes and they all have 401/9254) You can usually buy a 401 and a 9254 as a set which attracts a little discount on the servo. Be aware however that if you have bought a regulator to allow you to use 7.2v Lipo batteries as a reciever pack that it MUST have a 5v output (the align 2 in 1 does not). If you try to run the 9254 on 6 volts it will burn out. This is nothing to panic if you do have an align regulator though since they sell 5 v stepdowns for about $6.00 This fits inline with the servo AFTER the gyro. Feel free to pm me or post back if you want more info on this. Also to answer your original post, the futaba 3001 is a bit slow to use as a tail sero it therefore can't really react quick enough to the gyros instructions and therefore the tail has moved by the time the servo has gotten the tailblades to move thus causing the gyro to call for yet more servo.
Hope this helps... Azzy