Larry Beinfest had this to say to the press yesterday.
This is just trade justification talk. If all six play for the Marlins this upcoming season, not counting September call ups, something has seriously gone wrong.
Rabelo will make the team since the Marlins will carry two catchers. Miller will almost assuredly be in the starting rotation and Maybin will get every chance but he could easily prove he is not ready yet.
If we see the other three before September, well then, nothing is going right. That's not to say that one day they won't have a spot on the big club, but this isn't their time.