No issues with anything post-WWII. Assuming the prewar Merkels were built for the American market, they shouldn't be a problem either. The formerly 2 1/2" gun should be fed either 2 1/2" shells or lower pressure 2 3/4" loads. But in all honesty, German guns tend to be overbuilt. My first classic sxs was a prewar Sauer 16. Back in the Dark Ages, long before this BB and magazines like Double Gun Journal existed, I had the chambers and cones lengthened, and shot American factory ammo--frequently 1 1/8 oz loads for pheasants--for about 20 years. Nothing ever happened to the gun. The greatest risk is that it will either shoot loose (off face), or you may damage the stock due to excessive recoil (not due to pressure, but rather to heavier shot charges and/or higher velocity loads). You reduce the risk by shooting lower pressure and lighter loads at reduced velocity.
