Just in season 7 I believe the first only took the forms of Glory, The Master, Adam, Warren, Dru, Buffy & Cassie oh and that potential that the bringers killed, can't remember her name (blonde hair). After the season opener, and later in the season, the first took mostly the form of Dru (when torturing Spike) or Buffy (probably because it was cheaper to just write in the script that Sarah was playing the first instead of getting one of the other actors to come in and play the first).
Joyce, possibly, we just don't know "for sure" that that what Dawn saw was the first. Now, in Amends, the first only took the form of Jenny.
What episode did the first take the form of Spike in? I don't recall the first taking the form of Spike. Hmm, need to watch S7 again.
Question?