Hi AJ - your world sounds fascinating and I'm excited to see how it progresses.
Custom content refers to items that are created by someone other than EA that are intended to replace an EA item (e.g., hair, clothing, furniture, etc.) Items you find in any EP, SP, and in the EA Sims 3 store are all allowable. You are also allowed to recolor those items in CAS as you choose.
I hope that helps. Pam and Mods, if I have misspoken please feel free to correct as needed.