Some start immediately or at least quickly. They feel that getting some nitrate hogs into the tank early on will help with the cycling process, at least keep them down and keeping the fish less stressed if using a "live" cycle with fish. I am one of those that feels that waiting a bit is better. By adding plants early, I feel that having plants, which use nitrates as a food source, diminish the development of the bio-bed. Just my $.02.
And no you cannot, or should not, use the sand from the saltwater tank in the freshwater tank. The residual salt would, or could, be fatal to most freshwater fish.
The filtering system I suggested is a redundant system. Those redundancies allow for some neglect and failure of individual components. Leaving the tank alone for the weekend would not hurt. Feeding could be scheduled to compensate to your absence. Lights could be placed on timers so that the lighting schedule would be controlled automatically. I must say, that if you set this system up as we have discussed, many here will be in total envy. Including me. I have most of my tanks set up this way. It allows me to enjoy them and not be working on them all of the time. The only problem I would see, is leaving it for an extended time. Feeding could be done with timed auto feeders, should this become a necessity.
Water changes will still have to be done. Filters still need to be maintained. Plants will need fertilized. And fish will have to be fed. But, this has the potential of being one awesome tank. Planted tanks with driftwood always makes a nice presentation.
And if you really decide to go "full goose gonzo" you can add CO2 for the plants. Nice touch, some swear by it. It is effective. But, is not cheap, and it is not real necessary unless you intend to propagate some of the plants or have some kinds of plants.