Issues with the Foliage brush on imported meshes UE4.12

I imported an fbx mesh into UE from 3dsmax.
That will be the base for the layer so I nned to paint on the meshes of the trees and rocks I already imported from different FBXs.
The foliage brush is not working though!
It woks if I do the “paint bucket” but if I use the “paint” modality it won’t paint the meshes on the imported mesh; it works painting them only on meshes made in unreal such as, for instance, the Geometry box.
To me this looks like a bug of the software?.
Also, if I paint the trees on an inclined plane, the trees will be perpendicular to that plane but I actually need them to be perpendicular to the sea level (parallel to the Z axis) indipendently from the inclination of the plane I’m painting on, as trees grows on hills in real life. Where may I find that option please?
I can’t use the landscape tool because I’m working on a defined low poligones mesh, so the landscape tool doesn’t satisfy me for this purpose; I think the brush tool would, once working, still be the best option for me but I’m certainly opened to further suggestions on other approaches to the level design process.

Thank you all!

This is a reference picture of the result I want to achieve.