I am certain that guinea pigs can eat marigolds from time to time, but not too much because there is very little nutritional value. They can, however be used to remedy external injuries. I am not sure about your question, but I'm sure it wouldn't do any harm. If you would like more info about edible flowers for piggies, click here.
I don’t believe the forum supports home remedies as they’re often ineffective and technically mean you’re refusing to give your animal veterinary care when they need it. If your guinea is severely injured you should see a vet, otherwise, if it’s just a small scratch, washing out the wound with some sterile saline solution and keeping an eye out for infection would be much more effective.