When using the Implicit dynamic solver, I can use a conduit between a wetwell and a pump. With the GVF Convex solver, I'm forced to use a pressure pipe in this case. Why?
This is mainly a technical limitation due to the numerical solvers running behind the scenes.
The GVF Convex (SewerCAD) solver calculates pressure networks separately using the EPANET engine where all pipes are assumed to be full, so a pressure pipe is expected. With the implicit dynamic solver, all parts of the network (both pressure and open flow/gravity) are solved at the same time, by the same solver. The Preissmann slot method is used to simulate pressurized flow in both pressure pipes and conduits.
For compatibility between solvers, it may be best to use pressure pipes. Both conduits and pressure pipes can be designated as "virtual" as well (no hydraulic significance - flow in=flow out) for cases such as submersible pumps.