I started on Labor Day with four pullets that hatched May 1. Had my first egg six days later. I'm still very new to this, so I'm no expert, but I think to start and learn, pullets was the best way to go. For as young as the pullets you mentioned are, that seems pretty steep, but I guess that's not really for me to say. But I would guess you still won't be seeing your first eggs until the days lengthen in the spring. Still a lot more raising to do than not before production. I paid $10 each for my birds at point of lay. Maybe a little lucky to find that deal, but $15 at 8 weeks seems pricey.
That said, again, I started with four, and I'm a little concerned because some unfortunate chicken math (subtraction) has me down to two birds 6 weeks later. One, I suspect to a burning bush I was ignorant of (they are all being eliminated) and the other two an overly adventurous lakenvelder taking a walk in the standing corn at the edge of the yard.
If your coop supports more birds, I would say three is the bare minimum. Four was a perfect start for me.
Also, I would ask the breeder if he's wrong about the bird being a pullet if he will swap you for a properly sexed bird when it becomes apparent. If he's not willing to stand by what he sold you, I would be looking for a new supplier. Chicken or otherwise.