I never decide what types or even specific Pokemon I'm gonna catch beforehand. I don't have any plan, so to speak.
In Gen I, I usually took Charmander, sometimes Squirtle. In Gen II, I always took Totodile. In Gen III, I usually took Torchic or Treecko. In Gen IV, I took Chimchar (and had the other starters traded over). I guess I sort of have a tendency to choose Fire-types, though there's no way I'm going on a journey with a roasted pig - even a poisoned bulb and a mudfish beat it.
For sidekicks, I tend to take the early flying type (Pidgey, Taillow/Wingull, not Starly for some reason), but nothing else is planned - I just take whichever Pokemon seems cool and I haven't played in a recent playthrough. I'm not changing it up at all since I'm probably gonna do the same and have my party filled up before the first badge, as usual.
“These are my principles; if you don’t like them, I have others.” – Groucho Marx
