Good luck with it -- I hope it works well for you!
I know that when I've used mediums in the past, I always had to be very careful not to use too much of it, because sometimes they dry with a glossy, plasticky appearance. I remember a while back, I read a Joe Francis Dowden book on how to paint water, and I got a little gung-ho with the gum arabic when I was painting the reflections. Well, let's just say that my results weren't quite as nice as Joe's were... and I quickly learned that a little gum arabic can be a good thing, but a lot of it is a big disaster!
I think you can use the blending medium quite freely, as it's made to be used straight out of the bottle, but I think I'd try using it on a piece of scrap paper first, as a straight mix of paint and medium might take forever to dry, and it's better to find out on scrap paper than on a "real" painting. I'm usually too impatient to get on to the next step, so I'd only use blending medium if I were outside in the desert or something, or else I'd be sure to use just a small amount in some water to thin it out -- I can't stand waiting for paint to dry!