I wouldn't worry about a little mixing, even if there was you have still achieved what you wanted to do (i) use less expensive substrate (ii) and put it where you want it (iii) and you say you are using black gravel to the front of the aquarium so if your going for ADA Amazonia then you'll be doing just fine.
The only issue about mixing ADA soil with gravel or fine sand is if the color of the two are different.
Allot of tanks have ADA Amazonia at the rear and ADA Nile sand at the front so black mixed into white would look .