I'll admit I did think at first that the Destroyers could be the Imperium, but because of the focus on them being silent and black-clad plus the Orks and Tyranids getting their proper names in their descriptions I juked myself. Turns out I should have stuck with my initial read.
After all, if they were actual Destroyers, there wouldn't have been survivors.
As for the Marines... an OC Salamanders successor chapter, I'm reasonably sure. Vulkan for the Salamanders part, OC successor because I can't find any Chapter with a pauldron symbol of a hammer in flame. The Salamanders themselves technically have the right colour scheme, but the emphasis placed here implies black primary with green secondary, which is the opposite of what the 'manders have.
[X] BoomTek
[X] SaucerTek
[X] The Directorate
[X] Crude Bionics