The regular 1:10-1:6 thickness/diameter is just fine again for the secondary. Silver does have the problem of tarnishing and there's really nothing that you can do to prevent it from happening. Adding a clear coat over the silver will cause a second reflection at the surface of the coating as well as the silver so you will be getting a second image next to the main one. You will also have the problem of the coating not being even for thickness or surface shape so the second image will probably be out of focus/abberated to boot.
Silver is nice for short term work (1-6 months) but I will note that you can still get a nice image of something even if there is no coating on the glass at all as the bare glass will reflect about 4% of the light and that's enough to do a nice job of splitting the Double-Double with a 6" uncoated mirror.