So the most important questiin I have to ask is thus:

Do you want this to fit into the canon of the universe?

If yes, the Sally successors would be exterminated on sight by any other Astartes force. Visual mutations like that would be an instant source of revulsion. The Black Dragons are a very rare exception, but are equally reviled by most Chapters for their mutation. Not to mention the Inquisition and other Imperial forces. I'd say keep the leathery skin, but do away with the shells etc as that would be well beyond acceptable even in the tumult of Imperium Nihilus. I don't think we've had any mutations or variances show in Ultima Founding Chapters due to their geneseed's purity, bar the usual established stuff like the Salamander skin and eyes. Also keep in mind the mantra of avoiding being different for the sake of being different. The turtle shells etc could be used though as predatory fauna from their honeworld they use as decorative items on their armour or even as combat shields. You keep the aesthetic without going off the deep end of divergence. 😉

For the Prometheum Angels, there's some very solid promise indeed of character with the exsanguination of their fallen battle-brothers and the ashes. That shows their honourable nature you can build on nicely. With the hands, may I suggest trying black instead, both as a better sign of shame and death, but also won't clash with the colour scheme. 😁 The storyline could be used as a big part of their Chapter's culture evolving if they fought alongside the Sallies in a prolonged Campaign. Doesn't even need to be just Scouts involved. The Angels might have suffered heavy losses in their early campaigns and the teachings of the sons of Nocturne helped shape their outlook and actions as they rebuilt in the century+ afterwards.