The Thousand Sons are very callous with power. But that's it. 2/3 of the Librarius limit themselves as much as if not more than the Wolves. Also, Chaos is known by the White Scars as the yaksha, before the Heresy. They never made it a secret that they knew it as such. And the Wolves used the term "maleficarum" to point to anything Warp related, not done by their Rune Priests.

The loyalty of the Thousand Sons doesn't really matter. Magnus fucked up, but he still believed in the Imperium. Had broken laws, but were loyal in heart to the Imperium. But again, doesn't matter. Russ had a direct order, and he disobeyed. He later even says that he regrets that he was tricked, but that Magnus deserved what he got and he doesn't regret burning Prospero. He doesn't regret disobeying the Emperor's direct written order, just that he was used by Horus in the process of it.