I wanted to add my two cents, partly because I am one of the "Brothers Chickens". That is, I am the editing part of the two-person team that calls ourselves "Los Pollos Hermanos". I took Spanish in high school, and I understand the rule about putting adjectives after nouns. However, both "pollos" and "hermanos" are nouns, so that rule doesn't apply; the phrase can be said either way. It can be either "the brothers that are chickens" or "the chickens that are brothers". However, I am not a native Spanish speaker, so I couldn't tell you for certain. I notice that if I say "los pollos hermanos" and "los hermanos pollos", the former rolls off the tongue a little nicer.
Of course, as has been noted, this phrase is the name of a fictional restaurant from a TV show, so take it for what it's worth. We picked the name for our scanning team because we are both fans of the TV show that originated the phrase.
PS - regarding not encountering this word order in Spain, Mexico or Los Angeles, you should be aware that the two characters in the "Breaking Bad" TV show that created this phrase are from Chile.
