If the point is to be repugnant, then you have to make them really sad looking and smelly. Covered in feces, rotting body parts, fungi and insects, gooh and ooze, will fulfill those two criteria (make the oozes really smelly).
The poison (disease) is another direction, making them dangerous: to touch, to inhale their air, to listen to their voice (breath), to have their weapons make wounds and spill blood, to even look at them (poison fumes to the eyes). And resistance to poison probably has nothing to do with the human immunity system, it's mostly against pathogenic microorganisms (diseases).
What weapons should they carry?
1. Very obvious ones (for terrorizing the opponents).
2. Far reaching and multiple target weapons (for quick spreading the disease plus maximum psychologic effect).
3. Quick reloading, easy to make, carry and imbue disease ammo.
* I am not aware of any weapon that fulfills all of the above, but I'm sure anything can be invented when sufficient resources and engineer skills are available.
4. Light armor (if any) for quick movement, shields for protection from ranged attacks, spears for protection from cavalry charges, shortswords for melee.
What tactics should they employ?
I visualize them as a fast moving and flexible formation, like light infantry or javelineers, their main weapon being RANGED. They should have some capabilities to survive initial charges by cavalry and heavy infantry. Or maybe the rest of the army should protect them from them. After that, they should employ their psychological weapon to make them run away, but at the same time disperse to allow the use of their ranged weapons against any offenders.
Their biggest vulnerability will be enemy ranged weapons, which is why they should be very quick on their feet (no horses, the poor animals will be dying from the diseases very easily) and have some kind of shields.
Attack: go forward and disperse (probably under cover of rest of the army), rain disease to the enemy, fall back under cover, then join the fray at the SIDES (away from own troops!) for maximum psychological effect.