Pommels are never glued. Being an Apprentice hilt, the only things that will come glued on the Bastion are dummy switch covers if you order it empty (Initiate).
Sound on both for sure. I don't have any actual staff builds yet, but I do have a coupler that I play around with from time to time... lol.
The ASP couplers (1, 2, 3, 4) are not vented or really meant for staff setups. They're meant for spacing parts and hilt extensions, etc... though people have cut out the slots in Coupler 4 as DIY to vent sound.
The actual "staff couplers" (long, short, very short) are what you want, especially for an Apprentice hilt. The ASP couplers are the size of the Elite hilts and will be too wide for the Bastion body and give a noticeable (and annoying) lip/edge between the two parts.
Overall length is largely personal preference, but consider that a bo staff is about 71 inches on average.