Hi! I run both Muk and Empoleon too, nice choice
As for a 6th pokemon, my suggestion (and I don't think it's already been said) is that you find a decent grass type, to round out a solid fire/water/grass core so you have some natural type-switching synergy and stuff. I'm not entrirely sure what good grass types are available at your point, I know eventually you can get Leafeon, Tangrowth, Lilligant, Ludicolo (although he's also water type) Vileplume/Bell-awesome, and of course Roserade. So you have some options if you choose to go that route. Hope this gives you some ideas.
