The away fans tend to mix in with the home fans in the pubs these days, plenty about in the city centre and in the canton area, but i would stick to city centre if i were you, if you travelling down by car a good place to park is at GOL which is at the end of the road where Fitz Allan high school is, £5 to park but you get it back behind the bar, lots of away fans use GOL and its a friendly atmosphere, especially if you have kids as the owner lets them play on the 5 a side pitches for free. Be Safe.
Away fans quite often visit the Cornwall in Grangetown. Good atmosphere with lots of football talk. Google it.