Alomomola could have been a good option for slot number six, as one of its abilities allow the Pokémon to recover a decent portion of its health ever time it switches out, but this is in-game we're talking about, however, where we are freely permitted to utilize recovery items. Due to this, none of its might not be too necessary.
Personally, I would likely go with Swanna. Since it's partially flying-type, it could learn the move Fly without any block, which might be important to visit places. When it comes to battling, Fly could also be pretty useful; it's not a weak attack.
Aside from above, Jellicent would be another good choice, I believe. This Pokémon has good defenses, and could make a nice wall for almost any team. On top of this, Jellicent would have chance to withstand a lot of fire-type hits Serperior and Bronzong might be thrown, which is a plus!