I wouldn't mix it. From personal experience. It starts looking weird. I started with Tan pool filter sand sand, then some Aqua brand red sand, black, green, blue added. The substrate started giving off a weird amateur vibe, lol. I didn't like the aquarium look. Polys looked great as markings started to come out quite a bit. At the end, I just put in about 2 big bags of bright red gravel on top.
Go with what Dr. b stated earlier about just add enough to cover the bottom to see if you like it. I like that look of the red sand u posted from the allpondsolution web site. It might work out for u.