Toxin bonus to combine with Primed Cold into Viral.

PFastHands and Vile Acceleration are flex slots.
Flex slot options:
FactionDmg - Highest damage increase, but only works vs one faction.
PFirestorm - For +AoE.
Hammer Shot - Dmg increase, with a bit of bonus SC.
Vile Acceleration - DPS increase. Hammer Shot is probably superior for tougher enemies, while Vile Accel helps clear trash quicker.
Primed Fast Hands - Unusually helpful for Sustained DPS, giving a quite decent boost because of the slow reload of Zarr. But is ultimately mainly for convenience and/or clearing trash quicker.

Riven stat priority: CC = CD > Dmg > FR > Reload ~= MS
Riven replaces mod with same stats, in reverse priority; i.e: replace the mod with the lowest priority stat your Riven has. But don't replace [Galvanized Chamber] even if Riven has MS. MS is such low priority because GalvChamber brings a buttload of it.