Think about the volume and current strength you find in the ocean from the tide going in and out, it probably exceeds 90% of the flow rates most of us are keeping in their reef tanks. Saltwater fish are used to swimming in a current all day especially reef fish so you don't have to worry about them unless you are keeping pipe fish or seahorses and even they can handle more flow than most people think.
As far as your sand goes your tank may cloud up when you first introduce the new powerheads but it should clear within a couple of days. You will have to experiment with the powerhead locations so that you can get good flow across your sandbed and through your rock without excavating holes in your substrate. It will be normal to see some sand move when you introduce the new powerheads so unless the powerhead is really blowing hard on the sand let it go for 24hrs before you readjust it and see if the sand stablizes only move them before that if you find that they are digging large holes in the substrate. I have 2600gph in my 55g reef tank, my sand is extremely fine much smaller than grains of suger and my sand doesn't move. Its a fine line though because if I get a large mexican turbo that decideds to park in the sand for too long I get a sand dune.
