I would go with a bag of soilmaster select if you can find it. It is the same thing and you don't have to pay shipping. There is plenty of info about it on the board. You can get it from lesco or possibly another distributer for Oil-Dri products. One bag will give you good coverage and cost less than $20. I personally don't like mixed substates and do 100% of whatever I choose.
Yeah that will work, as long as the larger gravel is on top. All smaller sizes go lower in a tank, over time. I did that with some Eco complete once and it looked fine.
BTW, do you have pretty hard water? SMS will most likely radically lower the pH/KH/GH. Which is good if you have very hard water. If you don't, something more inert like Turface Pro League, or Flourite might be the way to go.