Best caliber for goats?

You cant supply a commercial restaurant, there are regulations that need to be meet for processing meat for public sale. But you can ask them them about if they know anyone who might be interested on individual level, or for pet food.
As for grain weight that would be like me telling you what pellet to use for your air rifle. I have no idea what will suit your gun, the distance you'll be shooting, or the effect your after on impact. You will have to sort that out yourself. Projectile testing is something you seem to like doing so shouldn't be a problem for you.