TM,
OK, I'll take a stab at it.
1. I'd use a gasket.
2. A refill, after draining, should take about 26 quarts. When refilling, you'd want to add 17 or 18 quarts and then check the level using the proper procedure, adding oil as necessary. It's important to avoid over-filling.
While you have the pan off, you might consider replacing the filter.
When reinstalling the pan, I'd highly recommend using a torque wrench to tighten the bolts, to avoid over-tightening. Allison specs the torque at 10-13 ft/lbs.
Bob