I think 5 groups of 5 players with 5 rounds would be good, it's possible to rotate players so that nobody plays the same person twice in 1 "season"
Also 5 player games will likely be more tactical, be quicker, and also less likely to have a lot of players eliminated early and have to wait for next round...
Just some thoughts :)
5 player diplo is much easier to make and balance too :)
Well if you want to run all 24 players on one map I've just finished the Scenario I was making for this league, it's here if you want to take a look anyway :)