Most of my cresties are in 12x12x18 exo terras (for subadults) and smaller geckos are kept in medium- large Kritter Keepers/Herp Havens. I also only house one gecko per enclosure, it is my preference.
I have tried keeping them in the 18x18x24 but my geckos never ate very well. In the smaller exo terras they clean their bowls most nights! I am going to look into making some enclosures from plastic storage bins though. The exo terras are nice but such a pain when I am going through and deep cleaning.
I have no experience with the gargs or leachies though!