You just asked for U.S. Army Special Forces soldiers, but here’s what else I think.
Who would you recruit first?
The foresters/woodsmen/hunters should form the core, Also, brigands and the green-tight-wearing band of merry men. Brigands, like Kegan and his band of escaped convicts from Krull, the movie, can be offered pardons for their service, they know the AO, and how not to be seen or found. Also think like the recruiting campaign from the Patriot.
Soldiers trained in the basics of combat, must still adapt to wilderness fighting or the light infantry environment. Watch the Military Channel for Special Forces Assessment and Selection (SFAS) show. Not every soldier, even decorated combat veterans, is cut-out for this fight. Also, read the selection process from Inside Delta Force: by Eric Haney, which is also the inspiration for TV’s The Unit.
Definitely not youths, maybe except for minor roles or responsibilities.
Also, look for indigenous peoples or disgruntled populous.
Also Children of Hurin had a similar theme.
The Am. Colonial Rangers had similar themes, looking for hunters, trappers, and wilderness-types. Later, when more men were needed, they looked towards the light infantry units for recruiting.
How large would an individual unit be? Mission would dictate this, I'm sure.
This is like saying how much WATER I need. IDOTS-METT-T, from individual droplets to an ocean….it’s not about size; all about organization. Sometimes one or two, but next moment, thousands.
Wikipedia article on Strategy and tactics of guerrilla warfare
Also,
On Guerrilla Warfare, by Mao Tse-tung
War of the Flea, by Robert Taber
Special Forces Guerrilla Warfare Manual, by Wimberley Scott
How would a unit be armed?
I don’t have at my disposal a couple hundred people to experiment, so I have used the best simulator I have at my disposal… Medieval II: Total War. I have run several dozen battles: a unit(s)of Sherwood archers, the closest to a ranger unit in the game, with max experience, armor and weapon bonus VS. overwhelming numbers of rabble peasants, Viking raiders, and/or other orc-like units. More often than naught the units gets over-run or caught up in close combat and lose with lighter armor and weapons. Terrain and position help increase casualties amongst the enemy, but eventually, it still comes to close-combat. So, I believe in the combined arms approach. Again, IDOTS & METT-T, but I would like a little bit of everything: infantry, missile, & cavalry, … maybe not so much for the siege weapons but if I am taking a fortification, then yes..