When using molasses in the drench, I only use a small amount mixed with some almond milk or Ensure. I have used it when treating coryza and my chickens are all alive and well (except for the one who didn't get it.) When using grapefruit seed extract, you are going to be killing off the bad stuff, whether it be viral, bacterial, fungal, or parasitic and the body is going to expel this through the intestinal tract, so a little molasses could definitely be of benefit in helping flush out the toxins.
My input is strictly for those interested in natural remedies and my knowledge is based on experience, not expertise. If antibiotics are your thing, I strongly advise you go that route. The recovery of the chicken is the most important thing and you have to have confidence in the treatment method you choose.
You are correct about the confidence of a product: My knowledge is based on experience and expertise. My concern is for the health and well being of the OP's rooster as well. The OP can make the decision which route to take.