there's no specific rule on how decks are meant to be built. They can be based on anything, heck you could use 4 different types of pokemon in your deck if you feel it necessary or 1 if you want to keep it simple (though this isn't Magic the gathering, so the whole type/colour concept couldn't be applied in the same way).
The only thing you should keep in mind when building a deck is whether of not the pokemon within the deck can help support eachother, mainly through their attacks o poke-powers/bodies, e.g - pokemon "A" has an attack which deals 30 damage to each of your opponents benched pokemon and Pokemon B has an attack or poké-power which places damage counters on any pokemon. How do they help eachother? Pokemon B makes Knocking out multiple pokemon easier for Pokmeon A by further damaging the already damaged pokemon, speeding up the damage process. Though in some cases, 1 pokemon could usually fill the role of Pokemon "A" & "B.