I don't think a 15 minute flood cycle is enough. The plants will be taking up nutes as fast as they can during the flood cycle and I like to give them an hour to do so on each flood. One hour on, two off is what I've used for years. I do it 24/7 during veg and flowering cycles. It maxes the oxy too.
The stone in the res won't hurt a thing. It's not needed with an ebb and flow system, but it won't hurt anything. Each time the tubs drain, it brings in max oxy all the way to the roots because of the displacement of the water. As the water goes down, it pulls in oxy on every drain.
Dumping your nutes every 10 days is a waste of money man. My plants drink a gallon a day. I add a fresh gallon of nutes each day and NEVER dump my nutes. At flowering time, I change my gallon a day to the flowering mix and still don't dump my nutes. This makes a smooth transistion to flowering and wastes nothing.
Good luck to you man!