I wore pants, but bruised my knees on the wall anyway. Most people don't try to use their knees to help them up the wall like I did though.
I reckon shorts are fine, it looks like that's what Chris and his son wore in any case. I suspect minor scrapes are going to happen no matter what you wear, but seems to me you can avoid getting too scratched up on an indoor wall.
Haha, I've already forgotten how to rappel apparently, hence my suggestion last time that one of could go up and detatch the rope and then just rappel down. I'm sure if we ask gravity politely it will slowly lower us down the wall, don't you think?