I like Phoenixes, fits more into the idea of them banding together for survival to me. Sirens suggests they're just in it to cause chaos.

If Chimeras are initially super rare the idea of them multiplying somewhat come natural disasters makes sense to me. Presumably the crystals they generate can then be used to create more Deltahumans? That could suggest a huge value being ascribed to them, perhaps there are bounties placed on Chimera's heads from the start?