When you use soil-usually you will not need any added ferts. The nutrients in the soil itself as well as what the system will make as it matures is usually all that is needed-unless the plants show a nutrient deficiency.
If you use only sand and no dirt-I wouldn't go deeper than 2 inches and you will need to add ferts for both water column and root feeder plants when using inert substrate-like sand or gravel only.
Just to be sure-since you are posting "sand/soil/sand" You want to place the dirt/soil-then cap it with sand-You don't need to lay sand then soil then sand again.
For a 75gal-I would use about 2-3in of dirt and cap that with half as much sand. The cap is just to help hold the soil in place and you don't need as much of it.
When starting a soil based tank-it is important to start out with enough of the right species of plants to avoid problems and possible crash.
What kind of lights do you have, age of bulbs, kelvin, watts and planned photoperiod. I am sure you already have the correct lights since you have read the sticky on NPT's.
Usually stem plants are sold in bunches of 6-12 stems without roots-You can either plant the bare stem directly into the substrate or you can lay an object like a rock on top of it until it roots and anchor itself.
With rosette plants-like swords, crypts, sags, vals...etc....plant them in the substrate with the crown slightly above the substrate line.
Plants like ferns and anubias anchor them to something-you don't want to bury the rhizome-otherwise it will rot.
Floaters-need to float, you don't want to bury or plant the Hornwort-but you can drape it on something so it is more upright.
Look froward to pics....