A smaller pump with larger filter is always better than a smaller filter overdriven by a pump that's too large. The equipment gurus should be along shortly ( Pooldoc? mas985?) to verify this, but I would think a 3/4 HP is fine for that size pool. Even though it'll take longer to turn the total water volume over, the filtration actually should be better.

With a CYA of 150, then you'll need to get the chlorine levels up to 25 ppm, and keep them there by testing and adding more chlorine as often as you can during the day, until the green clears up. I would also keep the pump running 24/7, brushing the pool daily, and keep an eye on your filter pressure, backwashing as needed, but not until you see an 8-10 psi rise in pressure. Considering your CYA levels, I would strongly advise you to use bleach, or liquid chlorine. In 24K gallons of pool water, each 2 gallons of 6% bleach will raise your chlorine by 5 ppm, so you can use that as a guide when figuring your chlorine additions. As I said before, though--you're going to need to be able to test high chlorine levels to get this one cleared up.