If you find that you like the variety of graduated effects, you can add them to toxins that don't have them by using the examples given. The lethal poisons are easiest, in that you can just decide which category you feel they fit into, and then apply the severity based results for that poison type.
So just make Lethal Toxin A a nerve poison, and apply the results for nerve poison, with the lethal result from the poison table being the extreme result.
For the non lethal toxins, I select a basic type of toxin and use the time to effect of that poison type, then I've gone one of two ways:
A) Make the max (non lethal) result of the poison the Extreme result, then create the lesser results (Often I use 50% effect for Severe, 25% effect for moderate, and then a weak result for mild like "dizzy" or "nausea" with action penalties.)
B) Make the max (non lethal) result the Severe result, and have the Extreme result be lethal (Even non-lethal poisons like anesthetics can kill), then make the moderate result a 50% effect and the mild result a weak result per above.
I used to use A, but then switched to B later, since at the time the PCs were using a lot of anesthetics to capture people, and I wanted there to be a risk of overdose/allergy resulting in death, even for poisons that were indicated with a max result of "Paralysis" or "Sleep".