Wooper would be a good choice. Seismitoed and Wooper will be key in certain battles. Water Absorb is best on Wooper and Seismitoed could have either Swift Swim or Water Absorb. I can vouch for Nidoqueen. The Queen is very tanky and hits back hard with Sheer Force Moves. Golem is a very good Sturdy Mon. This will save you sometime. In my opinion, I agree with dropping Sandslash for now for a Water/Fire type. I would keep Seismitoed and Quagsire in the back because you will need them later. Torterra and Bunny are very good as well.
Torterra / Nidoqueen / Diggersby / Golem / Camerupt, Seismitoed, or Quagsire / Dugtrio
Nidoqueen may not be as essential since this is a Mono Ground but she will be super useful in a certain Double Battle.