If inspiration strikes, do it. Counts as armies with strong themes are a great thing for the hobby.
Likewise, if you feel a set of rules suits what you want to play, use those. It's not like all those World Eater armies of yore weren't using Space Wolf rules that were simply better for the berserker horde feel a couple of editions back.
Funnily enough, me and one of my regular opponents have a situation where my semi-loyalist Death Guard offshoot uses the traitor
DG codex while his renegade space pirate marines use the loyalist
SM rules as those fit their themes better