Is it ok to hang a 12"H chandelier in my front hallway, with only 8 foot ceilings ?

Sure...or you can go with pendant lighting as well...chandelier, pendant,,,foyer swag lamp with chunk gl and black iron rectangular light is very suave and would accent the hallway nicely...as well as a simple chandelier...you can adjust chain length so it's not over bearing...There's some pretty cool looking pendant lights that are not hanging down so low but more focussing on horizontal expansion in their design...this would add some dramatic effect while still maintaining the integrity of your ceiling's height...so as not to insite clostrophobic feelings when ping underneath...my suggestion is you temporarily hang a windchime or mobile in that space and see if it is an overbearing feeling...just to "try it on"...before investing money, time, labor and the ensuing hole in the ceiling for electrical... in a foyer light...