I have no idea where you heard that too much in the way of return duct, return sizing is bad/wrong. Unless you have a high speed pressurized system, then that would be incorrect.In days of old the air handler would just be left open down in the basement, they were dirty but fine. You can certainly add returns to the individual rooms if you wish, i prefer that as opposed to the one or two large openings. Better airflow in my mind but it is not necessary. The problem comes with not enough makeup air, not enough/restricted return air. So whatever you do, ensure that the same square inches available with the current set up, that works. Will still be available after you do what you do. As far as balance, yes return access con play a role but it is more about controlling the air flow on the supply side. Ensure all supply duct flow can be adjusted.