It is not likely that the chickens will rid you of your fire ants. They may eat some but, the shear numbers would be overwhelming. Also, fire ants do not taste as good to animals as other ants. They have a very acrid taste apparently. In the event of a fire ant attack, your chickens would likely lose, especially if they are trapped in a coop with no escape. I would monitor the ant problem but, I would not deter from having chickens around them. I doubt many chickens fall to them. We have no fire ants here in Southeastern Ohio but, I am very familiar with them and their habits. Just be sure that you are not building the coop and run on top of active fire ant nests. If nests are present, use some type of extermination practice on them. Borax and sugar mixed together has rid me of many ants over the years without applying pesticides, not sure but, it may work on the fire ants also. Just keep it away from your chickens and other pets.