It IS reommended to dive the limits of your tank(e.i al80 80 + feet, 100's 100ft, 120's 120ft etc)
The limits of tanks are determined by the fill pressure and your SAC. Tank size should not necessarily determine depth.
Some divers can go to 200fsw on an AL80 with ease, while others will only go to 60fsw.
The "comfort zone" as you put it, is down to a personal style of diving which differs for each individual, and can differ with environmental conditions on the day - weather/sea state, etc.
I have seen AL80s filled to 3000psi, and also to 2500psi. That would influence my decision on dive profile.
Purely from a safety point of view it would be wise not to assume depth depends on tank size.
With regard to liveaboard diving, you will find that your diving should develop over the course of the week's intensive, but enjoyable, diving. As your comfort improves so will your SAC, your navigation, your buoyancy, and your appreciation of all that is good about our sport.
It will be an excellent learning experience on your first liveaboard, as will also be the case on your tenth.
Hope you have an VERY enjoyabubble trip!
Seadeuce