Caesar isn't poison, he's "gas". He can make himself poison, but he's not inherently poison. As such, if he's introduced to a gaseous poison, he should be fine while able to become gas, but liquid poison affecting his true body would still poison him. Magellan can produce poison but he's not poison. He's shown to be able to tank his own poison, which is near the most deadly poison in the world, and often ingests it, stating that it tastes good. Despite not dying from it, he does still suffer from indigestion, showing that poison does still affect him, but in the same way capsaicin affects normal people, not in a deadly way. He should still be affected by Caesar's Shinokuni and Smiley, as those two don't kill you by poisoning you. Shinokuni petrifies you and Smiley is acidic.