I usually use 50-50 perlite and coco, but with my new run I am doing pure coco again as I did in the past. I use playsand on the top to keep gnats out. Also, I line the bottom layer with coco, then put about an inch of sand on top of that. This makes it impossible for the gnats to get in through the bottom of the pot.
Gnats are a real pain if they can get inside your pots and lay eggs, as the larva eats your roots.