The bathroom is completely gutted. We're getting ready to put the PVC drain pipe in for the shower and it needs to go through 2 or 3 joists to join the main stack. I notice all the shower drains are designed for 2" pipe. To keep the joist holes as small as possible is it ok to transition from say a 2" P-trap to 1 1/2" pipe, or am I asking for trouble (slow drains) later on?

Also -- this is a 2nd floor bathroom and we don't want to open up the ceiling below for P-trap access -- I assume I need a one-piece P-trap (without the swiveling adjustable thingy) and need to be right on the money for exact center of the shower before we glue it together, before we put the subfloor down?