Second, Roblox has a 5,000 polygon limit, if your object exceeds that limit Roblox will not allow you to import the object.
To solve the issue with having a .fbx file due to exporting from blender, export as an .obj file.
To solve the issue with exceeding the 5,000 polygon limit, separate your exports into different files. Then create a folder to import all your meshes into and it should have the same object, just categorized.