I would be leary of european track...except Peco's stuff that's marked here. Some of the Roco comes with an integrate roadbed that will require a little effort to mate with "american" track. Also, the cast on details are for track bolted to the ties instead of spiked. As shaygetz said, their curve radii do not typically match up, nor due their track "codes" always match up. Then they also sometimes have a slightly different rail joiner set up...I don't know if it is compatible or not.
I'd recommend just getting Atlas code 83 with Atlas custom line turnouts (code 83) for cheap track and either micro engineering or shinohara (walthers) for really nice stuff. Code 100 is appropriate for modeling the Pennsylvania Rail Road...Code 83 is appropriate for modern mainlines...Code 70 is appropriate for most steam and transition era mainlines...Code 55 is appropriate for 1880 and 1890 model railroads.
You can also, when starting a layout, look into handlaying your own track. It isn't difficult, but it can be quite enjoyable. Turnouts are also WAY cheaper this way.