I think it would be nice to have an indicator for when you've reached maximum Skirmishers.
A toss out there whacky idea, which would be when flank is used, flank would be pushed off the skill list by another skill, which would be a spell book, which you would go into, and choose between different flank units, the cooldown of the spell book would be a tiny bit less than the time it takes for each spawn. And of course when Flank ends, flank will go back in its place and the spell book pushed off.