Based on how things played till now, if a character appears to be using a Pokemon, then we will face that Pokemon when we eventually battle them! This applies not only to Gym Leaders, but to other characters as well, for example Solaris, Sirius and many more! Unless Titania changes that and doesn't bring one of the four Pokemon mentioned, then there is no reason to think otherwise!
Klefki would indeed make a strong contender to be part of her team, but I doubt Titania will bring a non steel Pokemon for her gym battle! Unless your name is Noel, Gym Leaders stick exclusively to their typing in their gym battles!