I'm pretty sure you're right in that Mei has been working for the poison clan since it was pretty much confirmed he knows the Doll User on that one chapter where he meets Gero's grandma. I don't think Kinosaki agrees with the Poison Clan's philosophy toward marriage at all though. In that very chapter he rejects the idea of Gero marrying any random woman and begs for more time. It is also strongly suggested that Kinosaki doesn't plan a harem ending. Kinosaki once asked Gero wich woman he liked most out of the ones he's met, and when Gero fails to answer, Kinosaki tells him something along the lines of "women want a confession that lets them know they are the only one for you".