Go knock yourselves out. Importantly, Forge World kinda addresses whether you can use Horus Heresy Space Marine Legions in 40K. And while they basically don't give you really good guidance.
Q: Are the armies and units in the Horus Heresy books by Forge World meant to be used in
games against regular Codex armies, such as say Grey Knights or Orks?
A: While Forge World’s on-going range of Horus Heresy books and their game content are all designed to use and be compatible with the Warhammer 40,000 rules, they have been fine-tuned and focused on playing battles in the milieu of the Horus Heresy rather than in conjunction with the Codexes representing warfare in the 41st Millennium, and this will remain the case.
Designer’s Note: This means that while you are, of course, free to have fun and play games against your friends using any forces you like, and Horus Heresy forces will be broadly ‘a fair fight’ with Codex forces of the same scale, certain rules anomalies and inconsistencies may be thrown up that you have to deal with, although these should not seriously affect the game in most cases. (For example, certain units, such as those with the Stubborn special rule are at a premium costing in Horus Heresy armies over their regular Codex counterparts, owing to the results of play testing within their own sphere.)
Now, on the Apocalypse front, I can easily see allowing Horus Heresy Space Marine Legions in Apoc games.