Yes you can. How much would depend on how deep of a sand bed you want. I think I remember seeing a calculator for that. When I started my 135 I believe I used two 20 lbs bags of dry and two 10 lbs bags of live and it was like 3-4" deep.
I just checked my old build thread, I was way off.
80 lbs Carib Sea Ocean Direct Aragonite
40 lbs Carib Sea Live Sand
Yes you can mix the sand. How much sand you need depends on how deep you want your sand bed to be. I have a 60g I put 100lbs sand in my tank, because I wanted a jawfish. Dustin
Absolutely fine to mix live and dry sand. I really like the Aragalive special reef grade by Caribsea. I have a shallow 1 inch sand bed for looks as well as surface area for aerobic bacteria. I will stir and clean at least monthly.
You can achieve anaerobic nitrification if your sand bed is deep and not tampered with too much.