I tried looking into it online and all it really says is that they exterminated religious groups and had an idealized "master race" in mind, but I can't quite find out what specific type of people(s) they were trying to socially retain. Akin to how Adolf desired a nation of fair skinned, blonde hair, blue-eyed individuals, did the Khmer Rouge pine for the prevalence of a certain race or group of people(s)?
Yes, the Khmer Rouge idealized the Khmer people, those who spoke the Khmer language and form the largest ethnic group in Cambodia.
The exact nature of Khmer Rouge violence from 1975-79 is fairly contested. However, Ben Kiernan makes a strong case that cadres targeted minorities such as ethnic Vietnamese in Cambodia, as well as Cambodian-Chinese and Chan Muslims.
He also argues that the Khmer Rouge regime conceptualized their revolution as paving the way for Kampuchea Krom, a "Greater Kampuchea" that would include the Mekong delta region of neighboring Vietnam, based upon historic borders of the Ankgor empire.
sources
Ben Kiernan refers to violence against cambodian minorities in Chapter 2 of the Specter of Genocide from pages 29-41, interspersed with comparisons to other genocides of the 20th century.
Kiernan also talks about ethnic cleansing and violence against ethnic minorities in greater length in chapter 7 of The Pol Pot Regime, pages 251-312.