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>All pokemon on team are male (mono gender species are an exception)
>Only use regular poke balls for team
>Pokemon in team must be available in-game, preferably no traded pokemon (version exclusives being an exception)
That's about it. I also usually prefer to use the pokemon introduced in the gen (like only gen V pokemon in BW/BW2), but as long as the pokemon are catchable in the region then whatever goes, really.
Also, it irks me greatly if a pokemon I have on team has an evo I'm not fond of, especially if they evolve by leveling only. Pic very related.