Hi there !
For SolidWorks, I guess you checked the option setting for STL files … where you define whether they are imported as surface body, volumic body or just graphical representation (which you can’t do much with …). But you’re right, if the STL has too many faces, then Solidworks pops up an error message and loads it as a graphical volume instead.
Depending on geometry, I often have to cut pieces using NetFabb studio (free) into sub-pieces which are easier to handle (and SolidWorks Boolean difference suggestion doesn’t really work here as we are not dealing with ‘constructed’ pieces but imported ones thus facing limitations on functions we can apply).
Hope it helps