Ok, much clearer now. The extra bio-load of the tadpoles probably caused an ammonia spike, unbalanced you dpond and killed your fish. It sounds like things are on their way back to being in balance, give it like 2 weeks before adding any more fish. If you want, use a net and catch as many of the tadpoles as you can, and let them bo in a nearby pond or stream. too many can foul your water, as your experience proves.
The suckers are Plecostomus, most of us would say Plecos here on the board. just an fyi.
You definitley want plants in there, for shade as you said, but they also use up nutrients caused by fish waste, thereby blancing your pond naturally. Put as many plants in as you feel comfortable with, while leaving at least 50% of the surface as open water. Be careful buying water lilies and such, some of them get huge, way to big for a 50 gallon.
Dont feed your fish too much, want to leave them somewhat hungry so they will hunt down all the mosquito larvae and othe rbugs tha naturally want to live in every pond. Also, more food = more waste = more algae and green water.
Hope this helps.
CC