A 4th clan potentially takes a lot of maps out of the equation, a good number of maps on CC become total lotteries with 8 players, but whatever, plenty of other maps to choose from. Also not sure just how having a 4th clan will prevent "ganging up", instead of potential 2v1 it can be 3v1 or 2v1v1 - although I would hope that everyone would try to win each game for their clan rather than sacrifice their own chances to stop another clan winning another game, but it is the same as tourney play, if you can't win any given game you will do what you can to help the overall position if there's more to it than just winning/ losing the game.
30/50 round limit for all games, no problems with either as most won't go anywhere near that, personally I prefer to win games by wiping out the opponents/ gaining any objective rather than win by troop count a couple of moves early, but a win is a win.
Trench 50% max, fine.
Nukes, as long as no deliberately timing out is permitted then no need for limits, use at your own risk!
If there's to be 60 games then that's 120 playing slots, there's probably a formula but without one that says different I'd say min 12 players, max 15 games/ player.
I'm not in favour of one clan home set all at the same time, disadvantage for first to select, advantage for last, especially if they're leading.
4 clans - 5 each for 3 sets
3 clans - 7 each for 3 sets (yes I know that's one extra map/ clan, I like symmetry!)
Maps - I use for each clan, fine.
Start date - 3/4 days notice is enough for me to get it planned, so whatever suits everyone else is no problem.
FISO - home of the winners, FPL, SDT, TFF (unfortunately none of them play CC :-))