Post pictures. Some black dots are scabs from confronting another chicken.I have noticed my roosters crown having black dots on it. It has little chunks out of it from gnats biting it. I will get a picture of him later if I can. He also has little bits of white going through all the crevices in his crown. Please help! I am worried they might also be mites along with the gnats. He rubs his crown against me trying to get it all off. He acts normal but I want him to be happy and comfortable.
Don't worry about the white.. that's usually just dry skin. I would not be applying oil for any cosmetic reason..
Avoid using DE, as it is very ineffective in humid conditions or once wet. Ingesting it is not harmful in small amounts but inhaling it can be.
The only bugs that stick to combs are stick tight fleas. Using something PROVEN effective would be the wisest choice.
There was a gnat problem a couple years back in the US that was suffocating the chickens they invaded!
What country are you in? Why did you lose that many guinea fowl?
Sorry for those losses! Welcome to BYC, I hope your flock continues to thrive!

In the US, permethrin based "horse fly spray" is very effective at treating MOST external chicken parasites.. including using a dipped cotton swab on the head.
I first and second this statement!Let me offer a tiny bit of advice. You do not need to take every single bit of advice offered to you. It can be confusing and conflicting, and a lot of it is not all that helpful.

Please get other opinions and experiences as there are a ton of bad advice on here given by people with only the best intentions of helping.