https://xcf.trigramreactor.net/master/article/STR_HUMMERBoth the Humvee and AH-6 Little Bird use agent weapons not craft weapons. The machine guns, miniguns and rocket launchers can be attached.
For a full list of the things that can be attached to the HUMVEE are: PKM, M-60, MG-3, FN Minimi, Stoner LMG, BlackOps Assault LMG, BlackOps LMG, Heavy Machine Gun, BlackOps Light Minigun, BlackOps Minigun, Scatter Turbolaser, M.A.G.M.A. Pulse LMG, RPG Rocket Launcher, UAC Rocket Launcher, Rocket Launcher, Advanced Rocket Launcher, Recoiless Gun
Realistically though I wouldn't recommend arming the Humvee or AH-6 Little Bird as you don't need to intercept anything until you have actual interceptors. The only thing the humvee could be used for would be to take down manor defenders but by the time you have manors your landing craft should be able to bypass them anyway due to their speed, and failing that just bait them to leave the manor then return to it undefended.