What you have described above is caused by the fact that there are really two different types of feature definitions, there are:
1.) Line/Point/Surface feature definitions that link directly to a DDB (GEOPAK) or XIN (InRoads)
and
2.) Survey feature definitions that are imported from XML/XIN.
The reason you cannot see Line/Point/Surface feature definitions from an XIN in GEOPAK is because GEOPAK commands can only link to a DDB for these feature definitions, and the InRoads commands can only link to an XIN. This is because the program maintains an active link back to these files to pull the current symbology settings. Therefore, any command that uses a feature definition with a Native Style defined is actually looking back to the DDB or XIN for the symbology for the elements it will create.
Survey feature definitions act a little differently. Instead of an ‘active link’ back to the XIN/XML, Survey does more of an ‘import’ of the feature definitions for use by the survey commands. This import allows Survey to use XML(SMD) or XIN in either GEOPAK or InRoads. Therefore, any Survey command that needs to place an element will look at the imported feature definition for its symbology.