We have been keeping Sumatra since 2017 and ran my first flock purely as free rangers once they hit the 2.5 month grow out phase. All cockerels got themselves culled by my intolerance of their behavior. Our 2018 batch is still with us one hen culled her self from the flock by apparently harassing the dogs(being a recently broken broody) in the wrong part of the yard and was sent packing. This batch had only one cockerel survive he was culled in Spring 2019 during the breeding season for spurring/scarring me between the eyes at the PM feeding time. I still like the breed but think I might hatch out a small batch of Ayam Cemani and mix them as I love black lusterous feathers in sunlight. I am not overly concerned about keeping my lines clean vs actually having roosters I do not need to always cull out.
Other genetic studies shown that Sumatras are the ancestor of Silkie Chickens.