You don't have to remove ANY of your champs, most bg's run lots of champs to guarantee field presence. If you HAD to remove one, I'd drop Def. Knight or your Harvester.
True, but it's all up to you. Harvester is NOT a good rune, and is only decent for his detection and norafarming with globes. If you can live without it, I would. Otherwise Defiler Knight is the pick. Also, like I said, you don't HAVE to remove a unit.
Say WHAT? Harvester i a great rune. Flying, cheap, seminora generation and detection. It draws a lot of attention for its cost and it can usually be sent off to far font to force people to split their forces and spend more attention on it than it warrants.