You create a new UnitClass by making a copy of an existing Unitclass and then changing the name (and any other values you may want changed).
And yes, it will keep the SEAL as American only. When you set the DefaultUnit to NONE, you are telling the game that there is no 'normal' version that is available to everyone. Then, the entry in CIvilizationInfos tells the game that America has a special version it can build.
If you wanted the Seal to be available to everyone, you would set it to the Default for that unitclass and then make the change Thomas SG mentioned above for america.