As said, I started this team with Zangoose.
Zangoose is weak to Fight only, so I wanted a Pokémon that could withstand Fighting attacks, and KO the Pokémon that would attack me with them. Ghost works for this, but Ghost moves are physical, whilst most Ghost Pokémon have a higher Special Attack. Not to mention that most movepools are quite barren. Flying Pokémon would've been another option, but due to secondary typings there's almost no Flying Pokémon that actually resists Fighting. This leaves the Psychic type. There are quite a few Psychic type Pokémon, and quite a few have a decent movepool. Since Alakazam was a Pokémon I've been wanting to use and a Psychic type with a decent movepool, I decided to add him.
Alakazam is weak to Ghost, Bug and Dark. The first I had covered, the latter I had not. Fighting resists both types, so that part was quite easy. For the second part I wanted a Pokémon with decent stats, decent movepool and preferably one I hadn't used before. Hitmontop fit withing this picture easily, amongst others, but the design made me pick it over the others. I mean, really, a Pokémon that spins on its head? Pure gold.
Hitmontop is only weak to Psychic and Flying. The Psychic part I had covered already. The most rational thing for me to do would have been to look for a Flying resist, but for some reason I decided that I wanted a Fire type. Houndoom is the only decent Fire type who's not a starter Pokémon. The other thing Houndoom added was another Psychic resist and acces to Dark moves, to KO Psychic types as well. Since Houndoom's Rock weakness was already covered by Hitmontop, I decided that I could afford adding Houndoom and hence it's on the team.
So at this point I had weaknesses to Ground, Water and Flying to cover. Lanturn, I decided, had resistances to the latter two, and would be able to hit Ground type Pokémon for SE damage. The extra reason for picking Lanturn was, as I said before, acces to Surf, since I decided that I wanted to be able to surf without having to use a HM-slave.
At this point I had five Pokémon already. Two attacking with physical moves, three attacking with special moves. And I had a Ground weakness to cover. This, combined with some feelings of nostalgia, made me pick Swellow. A fast physical sweeper with a Ground resistance. The added weaknesses to Electric and Rock are not a huge problem, seeing that I can hit those types for SE damage with at least one of the other Pokémon. So, having decided on my sixth team member, my final team was as follows.