I think that cage is a good rosyboa cage, especially for the following types of rosies that tend to be smaller on average:
Mexican
Mid-Baja (San Felipe, San Matias, Bay of LA, El Rosario, etc)
Desert and desert/coastal (Whitewater, Morongo, Joshua Tree, etc)
Nice thing about rosies and glass tanks - rosies do fine in screen-top cages (lower humidity) and on undertank heat. So that cage will work.
Some of the coastal rosies can get darn big, and an 18x18 cage may get a bit tight. Depends on the locality and your care. Rosies in general do well in smallish cages. Kings on the other hand need more room.
I think Variable kings (aka "thayeri"
are a good choice for a kingsnake for that size cage. Thayeri stay on the small side compared to many other kings.
Cornsnakes tend to get long, but they will also use height, so the 18x18x18 cage works if you have a ledge or big branch for the snake. Branches are essentially useless for kings and rosies, but a corn may use it especially if you heat from above.
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Mark