I'd say informally. Though depending on the Clan, how formal they speak to the Warriors would depend. Some of them probably speak formally in the stricter clans(Jags, Falcons), or if they slip into informal, they probably get shown contempt or worse, causing them to either a) speak formal out of fear, or b) speak informal out of contempt, probably getting the ire of the warriors. In some of the more "relaxed" clans(Wolf, Diamond Shark, maybe Nova Cats), they probably speak informal around the warriors, though even in those Clans, I'd say there's probably still some cringe from the warriors to hear it. But I could be wrong about that, ymmv.