We should keep in mind what fearless leader / moderator Buffy Summers wrote on August 18, 2014 in a thread entitled "27 things you never knew about Buffy the Vampire Slayer":
18. The continuity of Buffy's birthday was jumbled throughout the show. A season one shot of her school records showed her birthday as October 24, 1980, while another showed May 6, 1979. Buffy's birthday episodes were always aired in January from this point on, and it was decided by Joss Whedon that her birthday is January 19, 1981, which is the year that appears on her tombstone in 'The Gift'.
So Buffy's birthday is subject to continuity errors sort of like the date of Christmas - the only thing we know is that Christmas was most likely not on Dec 25. Buffy was no more like a Capricorn than Jesus was and a birthday in Taurus (May 6) is also unlikely. A Scorpio Buffy born on October 24 would fit the character - one girl I know with that birthday was a feared and respected martial artist in our Tae Kwon Do club. (BTW Sarah and Alison are both Aries.)