If you get good batches of ecco, there are a lot of small sand grain sized peices, almost like it is mixed with a sand. And again if it is a good batch the largest size isn't that big. If this is the case, adding sand will just make it more sandy and shouldn't be a problem. It will get mixed together though, so just let it be. I just added 5 bags of eco to a 55 gallon and it was a great batch and it quite "sandy" to the point where if i was thinking about doin what you want to do, i don't even have to! Orlando only sells the good stuff when it comes to eco so unless you can see it first, be careful what you buy if you are looking something that is on the smaller side when it comes to eco.
Another thing eco is really good to work with as far as not having to rinse and not clouding up the tank. I don't know how flourite sand is but if it is anything at all like regular flourite, it makes a mess when disturbed to the point i hate it.. and i only have 1 bag mixed in with 2 bags of red eco in my other tank and wish i never did it. This might not be the case with the flourite black.
Stevie D