I do not know about the snails.

Vinegar is great for cleaning off the hard water and when rinsed, will not harm your toads at all. Sometimes a razor blade is necessary to assist with cleaning off hard water deposits.

I have great luck with only spring tails and isos as a clean up crew. Have never felt the need for anything else.

I would not add a carnivorous plant, your toads would likely stress them to death and I believe (tho correct me if I'm wrong, plant experts) that they need quite a bit of circulation.

I have heard of people having issues with pheonix worms because the frogs cannot digest them properly. The fruit flies maybe with a few occasional bean beetles mixed in and calcium and vitamin powders should be sufficient.