Quote from ZZZzzzzzzz:
And how would you rule out, or prove that the absence of mammal fossils in the Cambrian period was not by design?
Compatibility with a theory is not the proof of a theory, it could well be a spurious conclusion.
It isn't a proof - its a criteria for falsification.
Scientific theories cannot be proven full stop. Atomic theory will always be a theory, the germ theory of disease will always be a theory, even heliocentric theory will always be a theory. The same is true of evolution.
The scientific method demands that explainations must be testable to be science. Every fossil found tests evolution, as potentially a fossil can do a lot of damage to the theory.
An explaination which can never be disproven no matter what is found cannot be tested. Intelligent Design is one such explaination. As you point out it would not be falsified by finding mammals in the cambrian, and neither would it be falsified by the lack or the presence of anything else. In fact nothing in the fossil record tests ID. That is why it isn't science. It is more of a philosophy.