Hen with Vent Infection??

I have a standard laying hen that is about three years old and is a great layer, but recently I have noticed that she has/had lots of droppings stuck to her bottom, so I checked it out and her vent is very moist, gooey, red, puffy, and her butt stinks! I have tried looking this up but I didn't find anything on vent infections. I think she has been like this for a while now. So I put her in warm water and washed her bottom off and put her back with the flock. I checked her the next day and her vent is still oozing and the droppings are still sticking. I watched her all day and noticed that the rooster is mating her a lot more than the others, so I am thinking that she is either getting mated too much or she has an infection. I checked the other chickens and several others also have droppings that are sticking to their bottoms, but they don't have oozy, pussy vents. However, my white leghorn does and she even has a really dirty bottom. So I have separated these two, but I don't know how to treat them or how to get the droppings to stop sticking to the others butts. If it is an infection, can it be transmitted to the other hens if the rooster is mating them? The rooster looks fine. Should I be medicating all 40 of them or just the two?? I have been feeding them chicken scratch, egg maker, a little dog food, oyster shells, a little bit of DE, and sometimes they get at my goat's sweet feed. Should I be feeding them anything else to prevent their droppings from sticking or is this natural? Any help is greatly appreciated. I can post pics, but it is really gross.

 

I have been giving them DE in their food, which is supposed to help with the worms, but not all types. I sent a fecal exam in the vets office on a different hen several weeks ago for a different issue, and it came back negative for worms, but that doesn't mean that the others don't have worms. Should I be giving them a chicken de-wormer other than DE regularly? If so where do I get this? Feed stores? My animals are all free range on an acre property in AZ. There is no blood in their droppings, and they have been eating and drinking okay, but some might be loosing weight (the reason for the fecal exam). I have been giving the first hen some Betryl(sp?) mixed in water with an oral syringe, but I ran all out and it didn't really seem like it was working. I forgot to mention that this first hen also seems like she has a respiratory infection. She is sneezing a lot, discharge is coming from her nose, and she gargles when she breathes. So that was the main reason for the antibiotic Betryl. I guess I can send in another fecal exam or maybe even take her to the avian vet, but I don't have all the money in the world right now.
I really don't want to separate the rooster because he keeps the hawks away, and the hens really like him around, but if I have to, I have to.
I heard that molasses in their water helps with runny droppings??
Is there such thing as a vent infection/ avian yeast infection or do you think she has worms? Does anyone have any advice on a type of antibiotic that I should be using if it is an infection or a type of wormer? Thanks for your help!