In theory, yes you can land inside without the platform being raised, but some ships will encounter issues with this. The Sabre being an example, as the landed state retracts its wings, so the raised platform allows the Sabre to reduce its width before being lowered into the hangar. When landing on the hangar platform, you can expect that you will need to match the Polaris' velocity and bearing, as similar to how real-world mid-air refueling works on planes, it is a risky maneuver. You should generally expect good communication between the fighter pilot and the Polaris crew for a smooth procedure, or the safer option would be to either slow down or stop the Polaris. It will indeed be possible to take off while the Polaris is moving, but this is again done so at the fighter pilot's own risk.