Seismitoad is not that bad, but it's also not the best Water type to use. If you plan on keeping him, Swift Swim is definitely the better ability to use, mainly because of Rain Dance being included in its moveset.
Dusknoir with the crest as a held item is good. It is more of an offensive Pokémon and the crest boosts its weaker moves(like Technician does). Shadow Sneak and Shadow Punch are boosted, so I would keep those two in its moveset. Other damaging moves that are boosted are Bulldoze and Rock Tomb and both TMs are available in the current version. An Impish nature is suitable because you won't use its Special Attack anyway, but you could also change it into an Adamant nature to further boost the damage output of its moves. Overall, I really like your team and I like the inclusion of popular and less popular Pokémon.