I get you, in 2005 some 'Feyenoord fans' still had a firm which was active in the city. I'm quite sure we don't have that anymore since most of these guys quitted (and instead trying to earn a living by selling books and other ridiculous things about their 'heroism'). Only a few polish blokes who fight in the name of Feyenoord in remote areas and on appointment with other 'hoolies'. The city has become more open and international and I have only seen fight scenes at the stadhuisplein area.
If you want to escape the business of Amsterdam I would suggest Delft for you, it's a little city on a 12 minute train drive from Rotterdam Central Station and it is as old and beautiful as Amsterdam but less crowded and has quite a few cool bars. (The best for big groups is by far the Bierfabriek).