Firered and Leafgreen basically retcon the Gen I storyline anyway, so this is a little moot. You can't even find any more than the original generation's Pokemon in FR/LG until you hit the Sevii Islands or grab the National Dex. Also, your Pokemon that could evolve into a Pokemon of a later generation didn't. They'd stop evolving as if you pressed the B button. Perhaps at the time of Red's adventure, only 150 Pokemon were discovered in Kanto, but new species were found in the Sevii postgame, which was not included in the original Gen I, and would've occurred slightly after RGBY's storyline.
The Brawly/Bruno connection does not necessarily imply that Bruno would know anything about the Hoenn Pokemon. Brawly could've hailed from Kanto and then settled in Dewford later.
I'm personally hoping for a before Gen 1/Gen 3 placement in the timeline. We unfortunately see some Unova Pokemon in the trailer, so that could mean the this new region is just as far or farther from the Japan-based regions. That would explain why Gen 1/3 only had discovered 386 Pokemon, as this region is so far away that the newer Pokemon that appear here wouldn't be an issue. The XY postgame could happen contemporaneously with the beginning of Gen1/3's adventures, opening up trading if the player beat the game in XY to a possible Hoenn remake.