The sphere and cube are displacable objects, like the planet of TG. So they can handle displacement by PF very well. Imported objects are not, and it depends on the number of polygons whether you can displace well enough. Also, imported objects (with a lot of polys) that need to be displaced need to have 'force displacement' checked, or it'll be just bump.
Displacable objects can not (yet) be populated.
If you want to make a population of an imported object, you have to add a new population and choose your object as principal object. But as said, that can't be really displaced unless it's very high-poly.