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