The two kinds of flake should have very different qualities. It won't hurt the food to mix them but where would you use the resulting food? Goldfish are very much vegetarian in their preferences so a good goldfish food will be heavy in vegetable protein. A tetra type flake will be high in fish / animal type protein instead. In a pinch you could feed tropical flake to a goldfish and vice-versa but it would not be an ideal diet either way.
Green water algae are a good food for very small rotifers that can live in aquarium water. Those in turn are great first foods for extremely small fry. All algae use up plant nutrients as would larger plants and lend a hand removing ammonia and nitrates from the tank water.
