Yes, properly selecting/sizing your turbo is probably THE most important step of turbocharging your car. It isn't just the turbine size either, there are different housing sizes and not all kits are created equal. As for what gears, that all depends on your driving habits and where your power comes on. If your setup doesn't get into the powerband until later on you'll want steeper gears, if you've got a setup that starts really building at like 2800rpm or so you might want to go lower. Its all in the combo.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.