Yes, even though the meshes were made in version 2.91, you can still use them in your classic version of Blender, the only difference is that instead of simply opening the blend file, you have to use the "File -> Append" menu, or "File -> Import" and pick one of the other file types available (FBX/DAE/STL/OBJ)

Yes, but only if you open the greebles via the BLEND, FBX or DAE formats, then each of the objects have proper origin positions at the base of the mesh.

If however you import the greebles via the OBJ or STL formats then the origins will be placed at world origin, that is simply a limitation of these file types.

No, the meshes do not have custom UV unwrapping.

The objects do not have materials nor any material slots assigned to them, so you can rest easy that when copying these greebles in your scene you won't also be copying weird materials alongside and won't make a mess of your clean material list.

If you have some specific greebles in mind that you just can't find anywhere else then by all means please feel free to message me with some details about what you're looking for and I may create those greebles for a future update to this pack or a new pack.

If during your use of these models you happen to come across some sort of issue with a particular mesh then please message me so that I may go ahead and fix the problem as soon as possible.