I'm getting worried, I'm at 6 island, and I'm worried if it is possible for my rival to 'beat' me at completeing the pokemon.
Is it?
What, you mean at completing the pokedex? Is this some sort of joke question?
You shouldn't be worried about how many pokemon your rival has, it doesn't affect the game in any way whatsoever. You should just keep on catching pokemon at your own pace, and not worry about it.
And you never find out if your rival catches all the pokemon anyway, so why worry? If you complete the pokedex, all it does give you some stupid certificate. And after you complete the Sevii Island quest, all your rival does is go back to the leauge. So it doesn't do anything to the game.
It's NOT possible for your rival to beat you at catching pokemon, the number of pokemon he has has nothing to do with the game, as the game is supposed to do with the pokemon YOU have, not the ones HE has.
Does that answer your question?
~And this is a pointless thread kinda...