I am going to be switching from ocean forest to either Pro Mix HP or Sunshine mix #4. The ocean forest has worked great, I just want a little more control over my nutrients.
You will have the same problems with gnats with promix or any other medium you are using if care is not taken. It was not the "soil" that caused the gnats. It was not letting your soil dry out correctly, allowing bugs into your grow room, not putting your new cuttings into an isolated area for a bit so you can make sure they don't have bugs, etc. You can get fungus gnats in rockwool if you keep it super wet.
It's not going to be about keeping your pots covered. The larvae of the gnats feed on roots and whatnot, not the algae. Algae is a whole other issue in itself. Let your media dry out between waterings, don't let bugs into your room, and make sure you isolate any new clones going into your room from another person.