Live sand is not a must...However I personally really like live sand. My sandsifting sleeper goby does as well.
You can use
rock from a FW tank, but be careful...some rock used in a FW tank may have trace elements of metals which is very bad for your saltwater tank. So I wouldn't use it, there is too much invested in saltwater to skimp on rock.
As for sand used in a FW, chemicals, medicines and bacteria are different. You could create a problem trying to use sand contaminated with stuff from a FW tank. So while I don't have any, and I mean any experiance with such sand...I personally wouldn't chance it. JMO