I would do 50% water changes every day until the ammonia and nitrites are down. I would use NovAqua Plus for my water conditioner. You can get this at places like Petco or Petsmart, but it should help. It may be better to just start over and put the NovAqua Plus in it. It helps to establish a good balance of nitrates, nitrites, and ammonia without having to filter it for however long it takes to get it balanced. But I would test it every week when you change the water (10% for filtered tanks and 100% for unfiltered tanks). When the ammonia reads anything over zero, do 50% water changes until it goes down and test the amonia before every change. If you do this, the tads should be fine.
As for what they eat, mine eat algea discs, but that isn't really enough nutrition for tads. I would probably feed them boiled spinach or lettuce suplemented with frozen (not freeze dried-can kill your tad by making it constipated) bloodworms. If anyone more experienced has something different, then I would go with them on it.
As for the snails, I have had very bad experiences with wild snails. They lay eggs everywhere and the eggs hatch into more snails that lay more eggs. I personally wouldn't keep snails. On top of this, they can really stink up a room when they die. Unless you want your house to smell like roadkill, I wouldn't keep snails. If you're worried about algea, don't be. As can be seen through my tads eating the algea discs, it's their natural food source and would prefer to munch on it before munching on anything else. They would keep algea at bay, probably better than any snail or fish in a pet store. But you also have to think that those snails also eat the algea and you really don't want overgrazing going on. It's like putting too much livestock in a field with little or no grass without any additional food. The livestock are going to try to eat the field out until they start dying off.
Well, I hope this helps.