a 300 gph
overflow is fine for that size tank. you only have to consider what components are in the sump. in other words, if you have a skimmer that has a 400 gph
pump on it, it won't be working to capacity if you are only feeding it 300 gph
a refugium is not cleaning water that fast that 300 gph
won't be able to keep up.
that said, i would purchase the protein skimmer you want, and then purchase the sump to fit it, not the other way around.
i would stay away from the rio pumps. all of the ones i had didn't last very long. mag drives are better, as are "quiet one" pumps.