If its true straw, it doesn't mold...if there's any green to it, it will mold like crazy. I think an awful lot of what type of floor you lay down has to consider, the size, and how many chickens and how much you are willing to spend, if your good with a shovel, or want to tend to it weekly. Mold doesn't do so good in the sunshine, so if your run is shady and wet....you'll get mold. Probably. I really don't want to spend time in the run unless I'm playing with the chicks, and then I'd like some clean grass to sit on, but if your chickens are always in there year round, you won't have grass. You'll have whatever your area has under the grass, sand, rocks,clay,dirt...hard packed from all those feet. I've seen some clever adaptations with screened boxes that allow grass to grow up through it, they can eat what you plant but can't scratch it up. My problem is keeping the chickens out of certain areas...so I'm not real concerned about runs, earlier I said my run was 500 sq ft....sorry, I lied its 900 sq ft. It would be bigger but i got tired of digging post holes. That's big enough for broodies, and i can divide it in half if needed.
Probably if its small run you can't grow stuff, but you can provide bugs, mulch. Just like your flower gardens. They methodically turn it over and prevent things like mold...I just think it works. The thicker, the better. Yeah it may mold and decompose, but thats what you want...chickens don't care about it. If your chicken eats straw, it may be craving green things to eat...mine eat weeds and grass all day long. They nip it off too, natural grazers. Alot of time I notice them plucking at fresh straw in the coop...I think they are picking bit off it...seeds probably. I do feed fermented grains as well...have never seen sour croup.