I have the HS56 in my 18t.. it works frine. 16inoz of torque is more than enough. Associated has a kit for the HS56hb servo that has mounts, servo saver, and full instructions. its part #21049.

Servo tape will work on the HS81MG but from what I've read and seen it will come loose after a little because of all the torque and movement of the servo. You have to cut off all the tabs from the HS81mg to get it to mount in the RC18.

I don't know all about the Airtornics servo. I just hear that people like the servo and it is a good replacement servo.