The idea of Hylians being descended from the Ooccoo is a bit of a stretch isn't it? Doesn't really matter how fast evolution can occur in the Zelda universe, that line of development from Ooccoo to Hylian makes little sense for two reasons. One, both Ooccoo and Hylians exist in TP, but that can be explained by really selective evolution of only a portion of the Ooccoo race I suppose. Two, Link is from Skyloft, and he is clearly not an Ooccoo, so it's more likely that Skyloft has a population of Hylians, maybe even living alongside the Ooccoo as both races are supposed to be very close to the Goddesses.
Now, the Deku Sprout explained a few things about the Unification War that took place before OoT, and I think it's possible that the events of the war may have been influenced by Skyloft. The King of Hyrule unified the various races in the kingdom, and we know the Hyrulean Royal Family is Hylian. Perhaps the Hylian race originated in Skyloft and Hyrule was originally inhabited by humans and the other races like the Zora and Goron and whatnot, all of which were fighting with each other and their conflicts making it easy for evil to take over. After Link saves Hyrule, perhaps the rest of the Skyloft's Hylian population descended to Hyrule to help with strengthening the land through unification. This next idea came from another theory someone had somewhere on this site, I forget who said it, but basically they suggested the Hylians interbred into the human race, with the Hylian blood dominating over the Human blood so that by OoT, most humans had more Hylian blood than Human blood (thus explaining the larger number of humans in TP since the Hylian blood would have had time to thin by then). That's a theory I can believe, so that's what I think of this matter.