@evol_w10lv : It sounds like you know what you are doing, and are aware of the limitations of the sensors required for an inertial nav system. Depending on your application and reliability/accuracy requirements, a GPS receiver could be much more reliable than a magnetometer to make corrections. Of course if this is an underwater nav system then GPS is not going to be an option. There, you might want to add gravitometers to identify known mass anomalies, which I believe the U.S. Navy already does to move our "boomers" around in a very stealthy manner.
BTW, gyros drift whether "standing still" or not. They also have angular acceleration limits if part of a high-performance mobile platform. The rate gyros we used with our B-52H defensive fire control system in the last century were caged and mounted in a 3-axis gun control line platform, which was also locked until a radar "lock and track" solution was obtained. Only then were the gyros un-caged and the analog ballistic computer enabled to steer the control line platform to aim the Gatling gun. The two-axis gun mount was slaved to the control line platform azimuth and elevation axes until radar lock was lost or the target destroyed (whichever came first). There was also a roll axis on the control line platform because in a real live-fire defensive situation the bomber pilot would be jinxing and doing whatever else he could so as not be a target. A typical defensive encounter only lasted a few seconds, hardly enough time to accumulate any significant drift for this application, but those Sperry rate gyros were also used in nav systems.
Sorry I can't provide you with any help on sensor selection.