What are some must-have Pokemon for rain offense teams besides the obvious Pelipper and Swift Swimmers? It seems that my team's matchups tend to be lopsided in either a good or bad way depending on the opponent. Steel and Grass Pokemon could benefit from the reduction in Fire damage, but which ones are best for an offense team, I have no idea.
Good intuition about grass/steel types: along with bug/steel it's the combination that rain helps defensively the most. The composition of your team is up to you to an extent; rain has much more room to breathe thanks mostly to pelipper. consider that you'll want to keep Pelipper and your swift swimmer alive long enough to maybe go through a couple of rain cycles. You need stealth rock; Excadrill is a nice choice because it's immune to electric and takes the common coverage attacks that most electric types use to hit Ground types very well. Mold breaker is a great ability. It seems very odd indeed but mold breaker scarf rocks exca is sort of cool for rain offense. Ferrothorn is a more orthodox rocks setter and really appreciates the rain; access to spikes is fantastic and it allows you to scout move sets a bit more comfortably if necessary. It can also run other utility moves like knock off; it's very important to have knock off on something (or somethings) specifically for two mons that can really mess with you: Chansey and Scarf Pheromosa.
That kind of brings me to something pretty important: a powerful wallbreaker that can function outside of rain is a very good inclusion. You can get pretty creative here but absurdly powerful, pretty fast mons go here: Specs Ash Greninja is great for this and can also pivot although it stacks another grass weakness and you need to get a KO for it to be useful. It's disgusting in rain though and doesn't necessarily need be activated: its presence will deter sacking which is usually a good move for offense teams playing against rain. Kartana is a great option because it has problems damaging fire types and can take out opposing bulky waters. Buzzswole works a lot like M-Heracross did. Lele or Koko work here too and yay Koko can pivot.
That's something that comes up a lot: rain offense depends heavily on the user's ability to manipulate momentum and maximize usable rain turns while minimizing the amount of blind switching. U-Turn and Volt Switch are great for momentum: a slow U-Turn allows you to protect your swimmer while chipping the target. This is why Mega Scizor is a fantastic rain mega. Tornadus-T is often seen on rain teams as it's fast enough to use Specs and can still pivot with U-turn. a 100% accurate STAB Specs hurricane is nice. There are many others.
Consider that your opponent's mons resisting water is not terribly important, especially if you're using Specs Omastar or Specs Kingdra. Look to cover mons that are immune to water or the few that can stomach rain boosted attacks: Gastrodon, Volcanion, Chansey, Alomomola, Tapu Fini, Ferrothorn, Amoong, Mega-Venusaur are all potential impediments that can severely dent Kingdra/Omastar/Kabutops/Seismitoad. Fortunately, most of these can be worn down by repeated hydro pumps + SR + knock off. Opposing weather is annoying but frankly TTar/hippo aren't as prominent and you'll probably account for both in the course of building your team.
Sorry for the long post: rain's a lot of fun to build (and I didn't really cover that much) but like every other type of team the best way is to build a lot of teams.
Hope this helped.