I'm curious about the mere 20 game limit for prospects before they have to be graduated to the pros. It would seem to me that you are setting yourselves up for failure in this regard. It doesn't take much time for a player to reach 20 games and could easily find himself sent back to the minors for a few years thereafter. You are either going to have several teams carrying a lot of dead weight or you are going to have a lot of good prospects on the waiver wire.
In my experience the point of farm teams is to reward patience and foresight. By putting a 20-game cap on things you eliminate both patience and foresight and create a free-for-all type scenario with prospects that I imagine you were trying to avoid.
In my leagues we allow for players to be cycled up and down between the farm team and the pros up until a certain games played cap upon which time the player can no longer be cycled. If you want to keep the player on the farm you can keep him there for eternity but once you call him up after the games played cap is reached then he must stay up. It's a little wrinkle that helps keep prospects like Kadri, Niederreiter, Kassian, Adam, Boychuk, Pirri, Ellis, Elliott, and Blum from becoming waiver fodder until someone gets lucky enough to pick them up at the right time.
Under your system the best prospects are probably going to be goaltenders since they take the longest to reach their games played cap so whoever has the best goalie prospects under your system is probably sitting the prettiest.