In an rpg it's fine. We fought an ornate Daemonhost lady thing and used a Death Cult Assassin for it. I'm the Rogue Trader and we didn't have a model for it so I use a Commissar. Sometimes for large groups of minions we don't even use models we just use beads.
As long as it's explained to the players what's what then it's all good.
As long as it's explained to the players what's what then it's all good.