Er, fantastic question – mussar teaches us to have rachmonus. It’s a good middah, and there’s no reason not to be merachem on goyim when it’s allowed, unless that person is known to be an active enemy of klal yisroel. To speak to your case, charging a goy interest at a lower rate if you can tell that he’s not wealthy would be totally fine. Charging him no interest… I don’t know of an issur; it’s assur to return a lost item, because you’re showing that mitzvos aren’t important to you and that you’ll do them for everyone, as rashi explains. Here, by interest, are you showing that the mitzvah of lending a Jews money isn’t important by doing it for a goy? Great shailoh…
But charging a lower level of interest is definitely fine.
As to where we should get a moral compass; we call it the 5th chelek of shulchan aruch. One achieves moral thinking by immersion ones self in learning, until his thought process is the result thereof – a ben Torah is so called, because he is literally the son of the Torah… It dictates ever move and every thought he makes. The bigger one is in learning, the more of this daas torah he will have. We can get glimpses of this from stories of gedolei yisroel.
As for Lichtenstein, he promoted haskalah and violations of torah, not to mention Hebrew nationalism. My rebbeim never called him or his ilk by the term rabbi. Rabbi hershel shechter, most of my rebbeim called him rabbi, some didn’t, so i do.