Poision system:
Each hit should renew duration of the poisoning to 15 seconds and make an already poisoned human more sick, up to a certain limit (5 for most aliens, 7 for basilisk). Sickness is a float value that is translated to damage per second by rounding down. Eg. if you have sickness 2.4, you recieve 2 damage per second.
Poison transfer efficiency (PTE) is the amount of sickness an alien can deal per second. Sickness per hit = PTE / hits per second, obviously. Each alien class should have its own PTE, dragoon < tyrant < mara < dretch < basilisk.
If you hit an armored area, the sickness of the human is increased by (PTE / hits per second) * 0.8 for helmet or light armor, or (PTE / hits per second) * 0.6 for battle suit. Note that the alien should still lose a full PTE, so some poison is wasted.
If a human is poisoned above the normal posion limit (which only basilisks should be able to do), medkit only heals the normal poison limit, leaving the human with reisudal sickness.
Posion capacity is a total amount of sickness an alien fully loaded with poison can deal to a naked human. Each class should have a different one, dretch < mara < basilisk < dragoon < tyrant.
Direct contact with a booster pumps poison into the alien at a fast rate, eg. 10 per second.
Poison slowly deteriorates, eg. 0.2 per second.
Basilisk:
Generates its own poison at a slow rate, eg. 2 per second (advanced only). If in direct contact with a booster, the booster's supply adds to its own poison generation.
Able to give its poison to other aliens by attacking them with secondary attack. If aimed at another alien, it shouldn't shoot gas, but transfer poison instead at booster's rate. (advanced only).
Poison increases human sickness up to a greater limit than normal posion limit (7).
Swipe should cause direct damage, poison AND heavy disorientation when the sickness goes beyond the normal limit (advanced only).
Gas should transfer poison with somewhat lower efficiency than swipe AND cause mild disorientation immediately (advanced only).
Humans able to protect themselves from basilisk gas by buying an air filter upgrade that is available for both helmet and battlesuit. Air filter could also reduce damage from acid tubes.