Two things about converting plastic sheds (no, I've not done it).
One, the walls tend to be hollow, and few people have tools specifically for cutting plastic, so most wont get clean neat edges AND there's no structural strength once you punched your holes - its like a hole in a hollow core door, the skin on either side just sort of flops around, with little to affix to. Cant use nails, can't use screws for anything with weight or tension, have to ether add a backing material or use machine bolts, nuts, and washers.
Two, some of the plastic sheds are "metal reinforced", essentially injection molded around a metal skeleton. The good news is the metal can be screwed into, the bad news is that on top of cutting plastic, you might also need to cut metal.
and unrelated, on those gambrel roof designs, if you slide the panels down some, you can create some sheltered overhang where you can place ventilation, then just add material at the crown. But its ugly, nothing supports those "floating" roof edges, so they will fail in time, and you still have to shingle the dang things. If I absolutely had to use one, I'd repurpose the roof plywood for almost anything else, extend the roof rafters to create an overhang with new lumber, then put metal on it. But at that point, you may as well do the whole thing from scratch on your own, because the roof is the most difficult part of framing the whole structure.
Also, the gambrel is for overhead storage space, which is likely wasted in a chicken coop.