Obviously there are some good leaders, and the irregulars and indians are well suited for the role. But beyond that I'm not getting a clear sense for the best way to use them.
I would think that to get maximum ambush effect you should have one or two regiments both sneaky types grouped together into a corps and with a leader who is a good ambusher. Three regiments in one corps seems like it is a bit much, but maybe not?
Next: usage. She he move toward an enemy position? or park somewhere in a province that enemies are likely to pass through? Aggressive, defensive or passive posture. Obviously Set to Ambush is a given, but there are many variations possible.
My observations so far are that, you can actually ambush when you move into a province set to offensive posture so sittint and waiting for them to cvome to you is not always necessary/