Not gluing pvc slip joints is a recipe for disaster, even in non pressurized drain pipes. Utilize union fittings, which are designed to be disconnected/reconnected multiple times, in strategic places that will allow for disassembly when necessary such as leading into or out of pumps and bulkheads. Standard flat fitting bulkheads typically have threads for a threaded adapter but pvc pipe connections leading into the adapter should be glued. If your pre-installed bulkhead has a slip fitting I would glue in a short piece of pvc with either a threaded adapter or a union coupling attached to it coming out of the bulkhead.
The exception to this is a Uniseal bulkhead, which is designed to be used with curved surfaces like a barrel. It uses pressure instead of glue or threads to hold a seal.
PVC glue once dry has not shown to be toxic in aquaria. If that was the case we'd have thousands of reports of dead livestock as nearly everyone uses pvc glue all along the supply chain. Not to mention to your home as well. :)
