Here is some info regarding the dewormers and their dosing for chickens:Is there fenbendazole for chickens, my horses get deformed but do you know if the horse paste is OK for chickens?
She was sold as an Isa brown but looks a lot more like a Rhode Island Red.
https://www.backyardchickens.com/articles/worming-chickens—medicines-and-dosing-information.73369/
You are right, she does look more like a RIR, and a lovely one!