For municipal utilities it is common practice to install Mechanical Joint Ductile Iron Fittings in combination with PVC Pipe, Ductile Iron Pipe and even HDPE. Currently it is not possible to create a Pressure Pipe network that uses mismatched ID Materials.
In my content catalog and Parts List I have MJ Ductile Iron Fittings and Pipes in PVC, Ductile Iron, and HDPE in all sizes from 3" to 42". If I choose Ductile Iron Pipe everything works fine, but if I try to lay PVC or HDPE pipe in my drawing, Civil 3d is not able to find a matching fitting. This is apparently rooted in the requirement of a fitting that matches the chosen pipe in size as well as material.
Autodesk wrote under this link as follows. To Create a Pressure Network That Contains Pipes and Bends
5. On the Layout panel on the ribbon, specify the size and material for the pressure pipes. The pipes and bends will share the same material and nominal diameter.
6. Begin selecting points in the drawing. Draw straight segments to insert straight pipe segments, or enter C for Curve to draw a curved segment. After you select two points to insert the first pressure pipe, the compass appears and displays the angles of bends in the parts list that match the selected size and material.
While this requirement to match materials is useful with PVC pipes and glued joints, it is a complete nuisance when using common Mechanical Joint Ductile Iron Fittings because there, the material does not have to match in reality. Only the Diameter.
My suggestion would be to relieve the matching materials requirement and instead introduce a matching connection method requirement. This entails a second change because PVC pipes are usually interconnected via Bell and Spigot and mechanically to the MJ Fittings. Pipes in the content catalogs should have a dual connection option. This means a pipe in the content catalog should be created with 2 connection methods, which are either or. This way a pipe can, for example connect via Bell & Spigot to another pipe OR via Mechanical to a fitting.
PS: Mike_ADSK (Autodesk Support) is aware of the situation and could possibly explain further.