You can mix Eco with regular substrate. The literature that I have read states that Eco Complete must be at least half of the substrate if you are going to mix it, while still keeping its benefits. Personally, I like the look of Eco by itself and I would not mix it. I would let the plants have all the benefits of Eco without compromise. If you ever wanted to separate the regular gravel out later, it would be almost impossible, as you can imagine.
The grains range from small pebble-size to very fine sand-size. The finer particles settle down over time and provide an excellent anchor for the plant roots without compacting them. The larger pieces remain on top. I would not call any of the pieces large and rocky.
A 20 lb. bag will probably not be enough for a 10 and a 5 gallon if you are using only Eco. I used one and one-fourth bags in my 10 gallon. In all, I think I used three bags up totally for one 10 gallon tank and three 5 gallon tanks. One bag went completely into the 10 gallon. One bag went completely into two of the 5 gallons. All three tanks needed a little more, so I got a third bag. I topped off the 10 and the two 5s, and then filled up the third 5 gallon.
I can't imagine you not being happy with it
It is very pretty, my plants grow very well, and it is not dusty at all, when setting up the tank or when trimming/replanting later.
PS. Congrats on your new job!