If you're willing to read this I have an answer as to why the cabal, fallen, and vex are possible( not including the hive). Vex: they are machines simple, our own technology will produce similar beings, without the extra dimensions or stasis. The Cabal and fallen are relatively simple to explain if you accept evolution. Because matter behaves the same in a particular state for example liquid metal behaves the same as h2o, and gasses behave the same regardless of composition and well solids behave like a solid all life bearing planets organisms would be subject to the same sort of elements so to speak. A fish swimming through say liquid methane would develope the same structure and techniques as a fish swimming through water simply because a liquid behaves like a liquid, same with flying species and more specifically hominid species which is why you see a biological structure similar to ours. Life would be exposed to the same challenges no matter how it came to be.