Flash grenades sound reasonable, but I don't think they should do stun damage. Rather, they should impose an accuracy penalty for anyone looking at the grenade when it goes off (or perhaps more simply, units that are too close, period).
Yeah, we should probably add stun rods, too. We have melee combat anyway (knives). I never used them in the original UFO, though, stun grenades were much more effective.
I don't like the idea of anti-psionic tech. Psionics are, by definition, an act of willpower. Machines don't have a will. Psi-amps (and psi implants which we might introduce) just enhance existing powers, and psi helmets inhibit them, but you can't generate or extinguish willpower in someone with a machine.