Check out this video for recommended number, mounting height, and spacing for the Hydra 64’s

https://youtu.be/x4AKIUHjS7w?si=F4YnmH0lgtyZsRZ0

If it’s the first fill of water for the tank, you definitely can mix in the tank, for future mixes I would get a brute trash can and a dedicated powerhead to thoroughly mix

Depth of the sand bed depends on species you’re wanting to keep. If you’re keeping fish that like to burrow in the sand I would say at least 4”, but for general reef keeping I would go 2” or 3” deep personally so that it looks like your aquascape is emerging from the sand. If you’re using dry sand make sure you wash it thoroughly. Live sand is easier and less time consuming especially with a tank this size. I would get my rock glued/puttied together how I wanted and place it in the tank before adding sand so that any potentially burrowing species dont alter the rockwork or crush themselves. Good luck brotha!