I am assuming requiring SpecTableLayout after it is incorporated into Spec?
There is currently no configuration for spec since it is inside the image, so the only way would be to specify version of Pharo (i.e. it's incorporated in version X, so the user needs at least Pharo X) or testing whether the class already exists.
As for overriding behavior - loading a package would rewrite existing code.
There may be some issues if you were to concurrently update both Spec and your package, and then the latest would win; but there may be better solution.
Peter