Hi MC.As far as i know,slugs that have died through eating pellets are indeed very poisonous to frogs,birds etc.I would far rather have holes in my plants than inflict suffering to them.Can you not set up a better way of removing the slugs,eg beer traps.? I have had a nature pond for years and have the most incredible hostas,they get bigger every year.Yet i positively encourage slugs!!so that the frogs have enough to eat.You will soon get some frogs i am sure.Meanwhile i would try and remove every slug that you find dead,asap, to reduce the risk.
By the way,frogs will not eat snails but the birds will,of course.