Well...actually Lando told the fighters to split up in order to try get some of the pursuing Tie Fighters off his back

I certainly agree that the Rebels would've gained more support following the Battle of Yavin. But we didn't see Rebel capital ships in ANH simply because of where it was set. The only planets/moons we saw were Tatooine and Yavin 4. And I reckon the Rebels would've ordered any capital ships out of Yavin 4's orbit once they knew the Death Star was approaching.