If more realms than just the Reach and the Westerlands had fought at the Field of Fire do you think it might have gone differently, that they might have been successful in stopping the Targaryen invasion?

Hell no. That just ups the body count. If you’re trying to defeat Aegon’s Conquest you don’t do it with large-scale field battles, because that’s just a target for dragonfire. 

You do it with Dornish tactics. Split up your armies, abandon most of your castles (Casterly Rock being a noted exception) hide in wildernesses and inside populations, ambush his armies and then vanish rather than try to hold territory, stretch his forces out and then strike where the dragons are not. Wear him down and pick off Targaryens where you can.