First off, there is PLENTY of IC history to allow dwarves as paladins and hellions. I've only ever played one dwarf, but he got to master in Alshain's faith as a defender merc.
Secondly, there is jack-sh*t IC history for centaurs in either class, mostly because of what Cannibal said - all of their weaknesses just don't cover the advantages they'd get as a hellion or paladin. Can you image a centaur hellion or paladin getting hit with an opposed flamestrike? Centaur paladin would stand only a slightly better chance than a hellion, and it would still hurt like hell.
Ask anyone who's played a centaur priest. The buffs you get from your protection spells do damned close to JACK against any fire attacks, let alone opposed alignment ones. The hole in your armor makes a centaur paladin EASY prey for a low level cleave, while for a hellion, not having the extra slots for enchants would most likely be fatal.
etc. etc. ad infinitum
summary Dwarves, yea, centaurs? not a chance
If you wanna buff centaurs something decent, give them greatly reduced damage versus iron or something. The fire weakness is really the worst thing going for them, and I'd wager nine times out of ten its why they aren't played - and the tenth time it's for the lack of armor slots or crappy dex.
OR, make barbarians prime stat con. It wouldn't hurt the other races one bit, and it would give centaurs a nice boost.