Okay, this is tangential to the topic, and I know what you actually mean. . . but this statement seriously irks me. No, "if alien, then anything" is *not true*. There is not a binary dichotomy between "utter realistic setting" and "everything else." By introducing one fantasy element to a setting, you do not suddenly open the gates to any and all other fantasy elements.
Introducing a fantasy element to a setting, does nothing to the setting other than introduce that fantasy element. To use Man of Steel as an example, so you've added aliens to the setting. This means that having other aliens is an obvious and credible element, sure. You've also added alien technology to the setting, so having alien technology do stuff not radically out of line with what its already shown as being able to do? Also totally fine.
This does *not* automatically make mutants, mad scientists, or magic significantly more credible than they otherwise would be, because those things have nothing to do with the elements already introduced ( unless they do: a piece of alien technology mutating someone, or a scientist getting the collected knowledge of an alien civilization uploaded into his brain ). Each new fantasy element has to be judged separately, in light of prior established elements of the setting and the general portrayed "weirdness" tone. Introduce enough fantasy elements, and if written well, it may make the setting weird enough that any new edition is perfectly credible. . . but the same thing written poorly just means its *all* unbelievable, new and old.