300-600gph is just fine for a return pump; that's more flow than most skimmers designed for 90g systems can handle anyways. As long as there is enough supplemental circulation in the tank, there shouldn't be any problems.
As for recommendations, like Dogstar mentioned, Mag Drives are great submersible pumps (though they do have a tendency to build up calcium more than other pumps and need more attention to maintenance). A Mag 7 will fall right into your desired flow rate after head loss has been factored in, or any comparable Eheim pumps would give you excellent results.