I dont wana be That Guy but Fulgrim was a Daemon Prince when he poisoned Guillemam. Other than that I Love It I can just see Guilleman in his deep blue armor and Fulgrim going full noise marine and having colors everywhere bright and flashy all over his armor!
Chaos are those who've turned too or been tainted by the Chaos Gods (Slaanesh, Khorne, Nurgle, Tzeentch or all four of 'em). They (mostly) aren't demons (or daemons as they're spelled in 40k). Some particular powerful followers of Chaos are turned into daemons, and some full daemons can be summoned from the 'Warp', although the normal universe doesn't much agree with their physiology and they can't stick around for long unless they start bleeding the Warp into the real world or use some sort of Chaos Magics
You can read more on Chaos, and everything Warhammer 40,000, and its fantastic background at the Lexicanum
I like your version of the primarchs' armours. Most people tend to making them really slender, somewhat similar to Dante's or Sgvald's, but to my mind it doesn't fit. A Space Marine's supposed to scare the hell out of his enemies, not only by force but even by his appearance. And I for one couldn't fear an ornate beanpole too much... Or, summarized in a single sentence: they look awesome, man!