Eh. Just because there was a vacant seat prior to Ace doesnt mean Oden was planned, and by extension that doesn't mean Rocks was planned either.
Ace was given a a vacant division seat, thats the only motivation needed to really to create the vacancy in the first place. He could have retroactively used Izo and that seat to make a convincing story about Oden. Again, not saying he didn't plan ahead, he did plan Rayleigh that far ahead it seemed (though you could argue the same thing, that he simply used an old character design for Rayleigh).
For example, the impel down escapees could just be "blank" pirates Oda decided to release and forshadow, but hasn't tied them to anything. He could just retroactively make them all Rocks pirates just to even further solidify that he "planned" Rocks or is at least using his characters or plot points wisely