Been a long time since I've done this. Good to be back.
First off, that fourth spot should be Dragonite:
Dragonite@Weakness Policy
Modest/Multiscale
252 SpA/252 SpD/4 HP
Dragon Pulse/Draco Meteor|Hurricane|Thunder|Roost/Rain Dance
Likes the Rain, Checks Grass weakness particularly but offers good defense in general, learns Rain Dance: Everything you asked. Do watch out for Ice attacks, though.
I haven't played much 7th Gen but a quick visit to Bulbapedia tells me that Tapu Koko can attack physically and specially. With D-nite you have 3 special attackers and one physical, but you still want that Thunder, so go mixed:
Tapu Koko@Electrium Z
Naive/Electric Surge
252 Spe/128 Atk/128 SpA
Thunder|Dazzling Gleam|U-Turn|Wild Charge
Wild Charge will be your main physical damage dealer here, U-Turn serves the same purpose as Volt Switch but physical (and not STAB, but Wild Charge fixes that one, kinda), other two are the same, Nature and EV's change accordingly. Take this one with a grain of salt because, as I said, I'm not super familiar with 7th Gen.
Swampert is good, but replace Protect for Stone Edge- EdgeQuake coverage and do just a little about that Ice weakness D-nite brings. Pelipper is perfect.
Like I said, I'm not super familiar with 7th Gen and I'm just a little rusty in terms of teambuilding, but I hope this was helpful nonetheless.
-AlphaBurn