[Athens]

The length of the pin doesn't make a difference as long as it is long enough to hold the ball end off to the side so that the tension "jams" it up against the bridge plate.

If the pin is so short that the ball end slips into the space between the pin and bridge plate then you will probably damage the bridge plate.

The same might be true if pins with the wrong taper are used. You might want to check to see if the Martin pins are the same taper as the Guild pins.

BTW, how do you like the tone with the ebony pins? Every time I changed pin material on the Guilds that I've had over the years I always went back to the original plastic Guild pins. I don't know what kind of mojo they put in them, but any other material was a step back, at least in my experience.