This post will act as a rough FAQ for how 40k 7th ed and the Horus Heresy rules overlap. So feel free to post here any questions you have regarding the new rules and the campaign. You are welcome to suggest answers to your questions but don't get disappointed if I don't use them.
Q: Can I take an Unbound army?
Yes, but you need to forewarn your opponent that you will be doing so.
I would also recommend giving them a general idea of the theme of your
force so they can prepare somewhat (no one will enjoy taking on a pure
landraider army with an anti-infantry force etc).
Q: What Command Benefits do the "Battle is in the Age of Darkness Force Organisation Charts" get?
A:No command Benefits are used in the Secession.
Q: What rules are we using for Super Heavy Vehicles, D weapons and Apocalyptic Blasts?
in normal games use the rules as presented in the 7th edition rulebook,
in Apocalypse games use the rules as found in the Apocalypse
supplement. Alternatively use the optional rules for D weapons found in Legiones Astartes Crusade Army List book.
Q: Which psykers can take Daemonolgy (Sanctic or Malific)?
A: During the Betrayal, Eradication and Masscre Eras only models with 'Burning Lore' can use Daemonology (either). This is currently limited to Psykers with the Legiones Astartes (Word Bearers) or Legiones Astartes (Thousand Sons) special rules, or Imperialis Militia/Cults Rogue Psykers.
Q: What happens if a model suffers Instant Death from an attack with the Deflagerate rule?
A: The target unit suffers an additional number of hits equal to the number of wounds the character had remaining.