Yes, you can model your own tile and use it in the Roof Tiles Generator. There are some requirements for the model:

• Tile model should be manifold. A manifold mesh is a mesh in which the structure of the connected faces in a closed volume will always point the normals (and their surfaces) to the outside or to the inside of the mesh without any overlaps.
• It is recommended to place tile the way it "lies" on XY plane (horizontal) and its length is along Y axis.
• Tile model should be unwrapped manualy and the UV Map name should be "uvmap".
• Make sure Scale and Rotation transforms are applied.

Keep in mind! Higher density of the mesh - more time it takes for Geometry Node to calculate and build the roof. It is recommended to keep the tile mesh as low poly as possible.

If performalnce is low - convert roof with Geometry Node modifier to mesh when you are happy with the result.

If you want to change the shape of the roof - switch off Geometry Node for the viewport.

Not yet. This feature is under development. It should be released in the future updates.