I'm going to
Antar for more visibility, apologies if it was unwarranted.
Elo should not be the main ladder rating. In a contest of rating systems to predict the winner of 7,800 chess games, Elo came out second to last when compared to Glicko, Glicko 2, PCA, and Chessmetrics.
Here is the blog post for the halfway mark of the contest which explains some more:
http://blog.kaggle.com/2010/09/21/elo-vs-the-rest-of-the-world-at-the-halfway-mark/
and here are the final results:
https://www.kaggle.com/c/chess/leaderboard
Chessmetrics got 37th place with a Root Mean Squared Error of 0.70866
Glicko 2 got 67th place with a RMSE of 0.71747
Glicko got 68th place with a RMSE of 0.71760
Elo got 137th place with a RMSE of 0.74377
PCA got 152nd place with a RMSE of 0.76607
I don't know the feasibility of Chessmetrics being incorporated to showdown but Glicko 2 should be made the main ladder rank due to its higher predictive power when compared to Elo.
If Chessmetrics could be incorporated then that would be even better, their website explaining their formula is here:
http://www.chessmetrics.com/cm/CM2/Formulas.asp