Gardevoir is Psychic and would make an excellent addition to your team, I think. :) You can teach it Thunderbolt for coverage as well, which is nice. And Shadow Ball, even though Ghost is physical in this game, it's nice to have something to hit other Psychic types.
btw, you should evolve that Lombre. Go to the Abandoned Ship (you'll need Dive and Surf) on the route to the right of Dewford. You'll need to go down to the locked rooms and solve a puzzle, but you should get your Water Stone easily without having to do all of it. But you definitely wanna evolve that Lombre.
Oh, and I recommend being at least level 50 before going in honestly. Just use Gabby and Ty to grind levels and money! :)
btw, you will probably get better help if you make a thread in the in-game team help forum. That's where team help is supposed to go anyway. When you post there, just make sure you include your Pokemon's moves. It'd really help if we knew what you were running on all your Pokemon, haha. Good luck to you!