Whether you want to create a separate class or not I don't know. However, from a real-world military standpoint, physically smaller creatures would adopt different strategies than burly barbarians:
- Hit-and-run tactics and guerrilla warfare, to keep larger enemies off-balance
- Ranged attacks, to keep larger enemies from using their advantage in height/reach/strength.
- A greater reliance on traps and ambushes (much like kobolds), to maximize their home-turf advantage.
- High mobility, perhaps mounted like the Mongols
Iron Heroes had a class called the Harrier, which specialized in mobility and hit-and-run attacks. If you have that book, or can buy it cheap (maybe as a PDF), it might give you some ideas.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)