You absolutely can. You'd even have bits of scientific evidence in specific comparisons like Ashkenazi Jews versus The Rest (even if controversial) that is worth thinking about.

Ashkenazi Jewish intelligence

Despite all the hand-wringing, problematization of the definition of "intelligence" and furious rebuttals both statistical and fundamental, I find no prima facie evidence to reject the hypothesis outright, even if there is no compelling evidence to accept it. Which means it may be useful to make the assumption sometimes. That's the meaning of the word "assumption."

That there is no conclusive evidence one way or another, and that some people find the assumption distasteful, shouldn't stop you from making the assumption if it helps you work through specific interesting arguments and reaching contingent conclusions.

Just be careful and thoughtful about when, why and how you share those arguments with others, especially in public. "Intelligence," however defined (like height and looks, only more so) is a very precious part of personal identity for many people.

And if you ever use those assumptions in arguments that might impact the lives of others, just be very rigorous about it. Not only do you owe it to the people you might affect, your arguments are going to be put under a powerful microscope.

Personally, I've never found any interesting arguments that can be constructed using that assumption, but that does not mean they don't exist.