When is only said "leading" it means Leader of all generals. When it says "leading X unit" it means the class. Since Marksman's base class is Archer she still got the bonus, except Crossbow as said in the skill description.
When the skill says Archer/Infantry/Cavalry/Ronin it means that it will trigger with any sub-class under that three. Other skills will ask for specific classes like Tiger Shingen asks for Light Cavalry and Luis Fróis asks for Miko/Nuns