Those pokemon that Ash was offered for his Tauros, he could've used them. And why couldn't Ash just get a few more Tauros from Oak and trade them for the other pokemon.

By the way, could anyone list the pokemon the Ash was offered? In the book adaptation I had, he was offered a Nidoqueen, Rhyhorn and Onix but I'm not sure that's what it was in the anime.