FSA Shortcut Creation
The shortcut creation is driven by the Archiving Policy which in turn is set at the Volume or Folder target level.
It is not possible to control that only certain files will be converted to Placeholder (PH) shortcuts after being archived.
There are ways to manually create PH shortcuts by launching the Archiving Task in 'Shortcut creation only' mode, but this will be driven by the Policy.
another alternative is to use FSAUtility -c to 're-create' PH shortcuts, but this option would also be driven by the settings in the Policy.
One suggestion, if you have certain folders with specific content that would apply to your criteria, you could define this area of the file system as an additional target, which would then be controlled by a diferent Policy with the settings to create PH shortcuts.
I hope that helps answering your question.