Grapefruit are definitely possible in 8B - I know of several well established abandoned trees scattered around north Florida and south Georgia, including, I believe, Duncan, Marsh, Ruby Red, and perhaps an Oroblanco (pale yellow flesh, relatively few seeds, thick rind, large fruit, with the smallest the size of large commercial grapefruit and the largest almost the size of a standard pummelo, but based on its size -35 feet- it seems like it would have had to have been planted right when Oroblanco was first released in California). I harvest fruit by the suitcase (literally, collecting them in a duffle bag) off these trees every year.
I also have trees I planted, both in a 9A-mircoclimate (in town) and in a solidly 8B area outside of town.
For white grapefruit, I would recommend Duncan (or what I am assuming is Duncan - very pale flesh, very seedy, excellent flavor - fairly intense, but lightly sweet and not at all bitter). For red grapefruit, I would recommend Flame or Rio Red - very good, sweet, dark red grapefruit, without the bitterness of Ruby Red, for instance. I've only had a few fruit that were from a tree I could positively identify as Flame, but they seem to have a bit more acid tang than Rio does.
Pummelos are certainly less tolerant of cold, but there is the one tree I mentioned earlier that I really believe is probably Oroblanco that is 35-feet tall, never receives care, and seems to be almost bulletproof. My in-town pummelos have never needed protection, and my out-of-town Hirado Buntan and Oroblanco trees survived one of the worst winters in recent memory (though with significant damage) and came through last year's mild winter with only very mild leaf loss. No fruit yet (two years in-ground - the Hirado set some fruit but aborted them quickly in a brief dry spell we had this spring), but I am hoping that if we get a mild winter this year, the Hirado will fruit. Chandler is a good red pummelo - I probably prefer it to Hirado - but I have not had the chance to try a Valentine or Cocktail. I've never had a pummelo I didn't like, though.