Great idea!
About the alignment requirement: instead of non-good, I think it should be non-lawful. I can imagine a chaotic good rouge poisoning the evil king who plans to kill all humans in the universe...
But LAWFUL characters don't seem like the type to sneak poison into someones food

Also, rather than becoming totally immune to all poison, try giving the poisonist a high bonus to their saving throws against poisons. This means monsters that have poison still pose some threat if they are appropriately leveled.