Would this be a good fish stock?

No. A 20 high will not hold 3 full sized angels let alone the sharks. The angels will eat the tetras if they are of the smaller species. I have seen them eat neons and cardinals. Three angels will be ok for a while until they pair off then the pair will maim or kill the third. The 20 high is great for raising show quality angels because the fins will grow to the proper length. But in the end they are Cichlids.