I don't know about McGuire and Sosa, but Bonds and Giambi both had Grand Jury testimony that they took steroids leaked, and Palmeiro tested positive. McGuire did have the really awkward testimony before Congress ("I'm here to talk about the future, not the past") and Sosa became horrible overnight.
What you claim is not anything new. There are always rumors about 'roids, and that's to be expected in the age of offense. I really don't know. Bagwell's numbers were huge in the strike-shortened '94 season, especially when you consider that he played in the dead-air Astrodome. There will always be doubts about most power hitters, unfortunately. But the players brought it on themselves.
Can't remember which one of you is an Astros fan - so, I'm pinging you both.