ALL of us (I assume, but someone will be the exception) have separate files for Arch and MEP.
Unless you're working on VERY large projects, and are experiencing problems, you really should keep a single MEP file. Make it a central file and share it among disciplines (everybody has their own local file). This is how Revit is intended to be used.
Create different worksets to help keep M, E, and P somewhat separate.
IF you must continue on current path, you just need to make the correct families, with VA parameter editable.
Ed
MSKTD and Associates
Architecture Engineering Interior Design