i am by no means a pond expert but, heres a few points:
unfortunately, most turtles WILL nip at goldfish fins. they arent trying to be mean, theyre just hungry and looking for a meal. there are a very few instances where people have acclimated turtles to living with fish in harmony but, that is far and few between and shouldnt be attempted by beginners. ive done this very successfully but, in the end i had to separate my razorback musk turtle and my comet because i wanted to provide both with the best possible living conditions that i could. and them being from two relatively different worlds, this wasnt possible in the same tank. in essence, try not to mix them.
as far as the tadpoles go, can quickly turn into frogs but, a few wind up being waterdogs (a different morphalogical route than normal). my only concern is that tadpoles and frogs can carry a wide variety of bacterial infections and i believe a few parasites into your pond. it may not happen that often but, it something to think about. anyway, they may turn out to be very good inhabitants, just do a bit of research.
if you want to try controlling algae by biological means, i would suggest a pleco of some kind. being that most plecos can get rather large and wind up trying to suck on your slow swimming (or sick) fish, i would then suggest that you get one or two of these types: rubberlip pleco or a bushynose pleco. they will grow to a size of 5- 6 inches maximum. in a pond atmosphere,
maybe a tad bigger. if you winter your fish in your pond, you might have to bring the plecos in if your weather reaches temperatures of 50 degrees or colder during the winter.
good luck with your new pond!