VEVOR 8"x 8" Post Base, 4Pcs Stainless Steel Adjustable Post Anchor Bracket(Internal 7.8 x 7.3"), Adjustable Post Base for Pavilion Deck Porch Handrail Railing Support Decorative Anchor Plate

This base does not fit a 6x6 post which is 5.7x5.7. You have to trim .5 " to fit. Anchors are 3/8 as is the roto hammer SDS bit.

Hi buyer， Thank you for your time to the mail. The drill bit diameter is 12mm. Hope this helps. Best wishes, Jennifer Customer Support

It appears to be a 15/32 masonry bit.

Yes, I did so it could not rotate.

I have not but yes I do believe that you can drill extra holes in it

Yes it can.