I would rate Barrichello & Massa as both very good drivers, not in the same class a Schumi but both good enough.
I dont think it was the way he wouldnt allow a top driver to join him in the team, why would the team want anyone to replace Massa, Barrichello or Irvine at the time? They were doing a good job for the team and the team was winning. Many teams favour one driver over the other, that' has always been the case in F1 and will continue to be the case. Alonso got the best at Renault, I'm sure Hamilton got the best at Mclaren the past 2 season, despite what Mclaren say, Alonso has said the same during his time there with Mclaren.
Schumi is always favoured in teams and the team try to keep him happy at all time because he is worth that much to them. It's like Rossi in MotoGP, he gets everything he wants, why?, because he is the best and to be the best you must be have to vicious towards everyone else, you only get away with it within teams because you are the best.
Also when Schumi joined Ferrai they were lost. He & Ross and a few others built that team into what it is today. They turned it into a monster of a team. Schumi may have had the best car, sometimes he didint, but it was the best because he made it the best.
This post has gotten way to long