My knowledge of Pokemon games is very much cursory (i just, quite unexpectedly, really enjoy the cast and writing in this one) so i can't really offer any useful advice on what to catch. If you check earlier posts in the thread, there's a compilation of what Pokemons you can catch in each of game's hunting areas, so this might help you plan, hopefully?
Regarding the electives, the game is quite accurate with its recommendation to, at least as first, mostly stick to no more than 2-3 subjects and make your team from Pokemons which are compatible with these types. Once you catch up with your type studies, i.e. raise the level cap on these types to where the game expects your Pokemons to be at given point (in terms of levels) you can consider branching out to couple more types, for a more flexible roster.
One minor hint, which is a bit of a spoiler: if you don't plan to pick up any Electric type Pokemon other than your Pikachu, then you can pretty much skip leveling up Electric knowledge, and use that time on another type instead.