AECOsim Building Designer (ABD) includes every feature of MicroStation.
ABD SS4 includes, in addition to the Architectural, etc. modes, a MicroStation mode known as MicroStation Building Designer that loads ABD without any of the Building discipline tools so the interface looks identical to MicroStation. This is launched from the MicroStation Building Designer shortcut.
In the background the dgnlibs and other resources used by ABD are available so files created in ABD can be used in MicroStation mode without errors due to missing levels, line styles and so forth.
When creating Dynamic Views in MicroStation Mode:
Where MicroStation is used alongside ABD the site configuration can be designed to support both. The ABD workspace structure includes the Standards folder and other locations used by the MicroStation workspace so placing existing CAD standards into the ABD workspace will make them available to ABD users in any mode.
Modifications would be needed to an existing MicroStation configuration to enable plain MicroStation to read the level libraries and other resources contained in the ABD dataset if they are needed for MicroStation operations. The need for this will depend on the context. Any levels etc used in ABD will be stored in the files so the libraries, etc. may not be needed in MicroStation.
(PowerDraft can also be used alongside ABD in the same way with exactly the same configuration changes as MicroStation.)
Customisations designed for MicroStation should work in ABD but may need modification if they use configuration variables or other resources that are modified by ABD.
Customisations designed for MicroStation that run into errors with ABD's discipline modes may work as normal in ABD's MicroStation mode depending what they do and interface with.
In both cases it will depend on the features that are being accessed by the customisations. Thorough testing will be required in all instances.
AECOsim Building Designer - MicroStation Mode as Default for .DGN