Not sure never used it, an extra small bag cant do any harm I guess, it'll not add hugely to the overall depth, even if you find you have enough from the big bag. Might be worth it, save postage if you find you need more
The only potential way to stop the two mixing would be to use a gravel tidy, however I dont know how these work in planted tanks, as I would think there is the possibility that they restrict root growth and might stop the plants getting to the plant substrate below. However, if you want to be our guinea pig go for it!
You can get them here
http
/www.aquatics-online.co.uk/catalogue...-separators.asp
Sam