http://www.fishtankforums.com/saltwa...your-test.html
Click the link above and download the logbook that I created for keeping track of test results. (requires Microsoft Excel) On one of the pages in the logbook you will find a Sand Bed Calculator. Enter your tank dimensions (Length, Width, Height) into the Tank Volume Calculator, and enter "4" into the Sand Calculator. It will tell you exactly how much sand you will need to acheive a 4" bed in that size tank.
I could calculate it for you, but you will need to tell me the dimensions of your tank.
As far as mixing LS, its perfectly ok. In fact the more diversified your sandbed the better off you will be as many sands come with different micro-organsims attached.
Good Luck, and keep us posted!
-Aaron