Hi,
Please let the players to join the war themselves, NOT leaders and co-leaders put them in war. and also don't limit it to 10-15-20 (the minimum of 10). Then match it with another clan with the same no of players.
And if someone left the clan before the war starts, either leave it like that, or remove the same number player on other clan. (it can be optional) or some other ideas.
Would be a bad idea, you'll get over-enthousiastic newbies in you clan war, and there'll be no place left for the serious players (who might be online too late)