So, random thing, Rules question, because I just don't have every single codex and dataslate and supplement and expansion on hand...
I have a Space Marine army.
I want to add something big to it. A real 'Centerpiece' model. I know for Imperial Knights, you just get that codex and ally, right? What if you want something like a Baneblade? Or one of the Forgeworld things, like a Sicaran or a Fellblade? Do those count as 'allies' from a different rulebook, or are they considered as part of the Space Marines Codex and take up a certain slot?
Any help is appreciated!