No, a FP object cannot used directly as a RC segment. RailClone expects that segments are standard objects (with a mesh), but FP by default generates instances at render, not meshes.
The technique you use with RailClone as exclude object may work, but it's not very precise. There are two better alternatives:
Method 1) Create a mesh piece with the tile and grass attached, and use it in RailClone as segment. It can be done generating the sample grass with FP, but setting Display->Viewport to Mesh and collapsing the object. Then, attach it to the tile (this step it's necessary to create the multi-sub material with all elements).
The problem with this method, is that all grass patches will be the same. You will get a non-desirable pattern effect.
a) Generate the tile as usual in RailClone, but without the grass. Set a specific material ID to the ground area, which will use to scatter the grass over it.
b) Set Display->Viewport to Mesh and turn off Display->Render->Use Instancing engine. In this way, the RC object will be generated as a standard mesh.
c) Create the grass with FP, using the RC object as scattering area and set Areas->Surf.Mat.ID to the ID used for ground between tiles, defined in step a).
This procedure lets you to adjust the grass once generated, and visually will look better.
Please note that if you use high-poly tiles and large areas, the RC object may be huge because doesn't use instances. To avoid that, you would create two RC objects, one for the tiles with instancing on, and other for the ground as a mesh.
I hope that helps.