Eh.... Rival

[Mudkipchan]

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?

 

[Charizard15]

I don't think you really talk to your rival anymore other than at the Elite Four.
So you shouldn't have to worry about your rival beating you to get all the pokemon.

 

[lazyslothboy]

I don't believe so. After you defeat him at the Elite Four, now at that point he appears to tell you that you can make pokemon eggs and that you need to complete Celios machine to complete the national dex. Other than that, he is the strongest trainer, besides yourself, in the game, with his high leveled team. So to answer question, I am 99% positve that the answer is no. Of course, I have never beat the pokedex, but it would seem that that is the way to finally defeat your rival once and for all.

 

[Charizard15]

I looked at my pokemon guide and there's nothing in that says about your rival after everything happens.

 

[Mudkipchan]

Well, that is great news. Thank you.

 

[Manly Blissey]

In other words, you're safe dude.