The issue is that you cannot feed a germanium pedal with the same pedal power brick that you are using to feed rest of pedals, if brick's outputs aren't isolated.
If you are runing pedals just with batteries, no issues.
If you are running your positive-ground pedals (like a vintage germanium fuzz) in a brick (without isolated outputs) and negative-ground pedals (most of them!) in other brick, no issues.