Hey engracio! Welcome back to the original "Or Bust" project. We could definitely use your prime finding luck on this project! :P
Pretty much whatever worked well for you with Five or Bust will work well here. Typically I do half the workers and do two threads per worker (or 4 if hyper-threaded) just like you said.
Currently all second-pass work is being done by the legacy SoB client. I am not sure what you mean with the legacy client not having second/third matching residues, as that is what the legacy client is doing. All results are eventually double-checked. The difference between PG and us is that we have a large gap between when first-pass and second-pass tests are done (second-pass work is currently back in the 13M range) while PG does their double-checking right away. (PG credit is not given until matching residues are returned, so there's a problem with people having to sometimes wait weeks or months to get credit)
As far as I know there is no way to do second-pass work with Prime95 unless the queue is switched server-side. I don't know what the exact ranges are, but we are in the 19.4M range and PG has been working somewhere in the 18M's, and has recently jumped to 20M so I believe we have the rest of the 19M's.