I know that the scale is a problem but I also admit that it isn't something most space opera can handle well, either. Rather than just showing up and ordering the GM to change his setting I just acknowledged this problem and moved on. Hoping it would never become an actual issue. I agree that some flexibility is good but as somebody who pleaded for special permissions in the past I know it'd only set up problems.

As for consistency of tech levels. While that sounds fine on paper in effect it throttles creativity. If I were the GM I'd also give special exceptions to some rules when I see it does not hurt but I have no such authority. The reason why limiting the FTL drive to known variant is preferable because these devices determine the scale of the game. On the other hand if there's a reasonable alternative which keeps the scale, I'd personally allow it. But I do understand when a GM wishes to stick to the guns.