Four 3d6 bolts works for me.
Do we need to reduce the base damage of the natural attacks? I think the lower Str should suffice.
I was going to nudge their natural weapons down a bit, just to make them a bit more different from the females.
Nah, that just seems unnecesary, and a bit arbitrary. We can nudge the Str down more if you feel it's necessary.
A typical adult duru grows to x feet in length, stands x feet high at the shoulder, and weighs x tons.
The reduced Str is ok, sure. Size sounds fine, too.