There is a large asian community round the outskirts of the city center, but that's no different to any other large city in the UK really. The areas of Grangetown and Riverside (which is the main one found by walking over the bridge as you walk away from the main train station) are the most notable.
There's also Bute which is on the way to the Bay from the center which has a fairly bad reputation (used to be known as Cardiff's red light district), not often a good idea to walk down their on your own at night (since things tend to get a bit territorial) but it's a lot better lately than it used to be as long as you take the road that runs alongside the newer flats as opposed to the one that runs alongside the other side of the railway (you'll know if you ever find yourself walking to the Bay from town). Sometimes the Bay itself can be a little unfriendly but because it's been cleaned up lately it's getting better.
Can't say I've heard of too many run-ins. A few friends have been chased/threatened by Somali's in the past but that was 2 or 3 years ago and things have calmed down a lot since then. It doesn't seem to happen past a certain age, or at least, happens less often.
As far as being unfriendly to ethnic minorities? can't say I've seen or heard much of that either. Most people tend to be indifferent There CAN be some general hostility from the avid Cardiff City football supporters on match days if they're properly riled up, but otherwise it's not something that you hear about very often at all and that's not really from the Uni either, plus it's moreover directed at the opposition fans. Also, the match ground "Ninian park" isn't even in the city center, nor close to any of the halls either. So I doubt Cardiff is too different from most universities of its calibre in that respect. Where did you hear that idea from?
In most cases anyway, if anyone were to be giving another student grief because of ethnicity, a lot of their peers wouldn't stand for it.