I dont know how you exported IDF from Mentor. I know the Allegro. while exporting there is a popup window in which you have to select default package height, version, and filter(it excludes the selected items like route keepout,via keepout, pins,vias, board outline, route outline etc). Generally, we uncheck board outline and placed component for mechanical verification.
If mentor has such option then uncheck/deselect items you want to export. IDF file genrates with BDF file.
while importing in allegro, you need to brose this bdf file. this contains all information of symbol package, height and dimension.
Hope this is helpful.