Well nidoqueen is neutral to it (I always forget she's part poison).
If you want, you can test your team out on me. I switched the connection back to WEP and it's worked out for me (been mostly Gen Ving, and I d/c'd on the last move of my match earlier but it's much more reliable). I'm running a new team, so idk how good it'll be, but it's so much easier to put theory to use than to just talk about it.
Depends on the team really. I use both of them on different teams (though my nidoqueen serves as bit of an antilead because I carry taunt). Slash has better natural strength, and doesn't necessarily need to even be a rocks user. You could use him as a spinner, and focus on his offensive stats moreso than defense.