Can you do metal filled via?

=> Yes, we can do that.

If so, what are the rules for min via hole diameter, min via pad size and min hole to hole clearance?

=> The via hole diameter range should be 0.15-0.55mm. And,If via hole is 0.2mm, then min pad diam is 0.4mm.

The pad diameter should be bigger than hole 0.2mm at least, min hole to hole clearance should be bigger than 0.25mm.  This size is for all size(different size) of via holes(the hole edge to hole edge should be no smaller than 0.25mm).

And what is the metal fill material, is it copper?

=> The metal material use for via filling is copper paste which is mainly a combination of copper powder and resin.

What is the surface roughness of the metal filled via process? Is it same with non-conductive filled vias?

=> The metal material use for via filling is copper paste which is mainly a combination of copper powder and resin.

There is no detail datum for surface roughness of the metal filled via process, but this kind of process is copper plating for flat treatment, and the maximum dimple will not exceed 70um.