Thank You for the response Mark. I will play it safe and just use one source of sand.
I have mixed sands threw many of my pieces. I often put marble or silica in my face coat and use a washed sand or a pool mix in my backer as it has a more even gradation and I feel personally it makes for a stronger piece.
Guess you will never know unless you try it. You should probably stay with a bag mix I think Mark sells one.