As far as I know, there were no Hungarian and German setllers in Krajina (Banija, Kordun, Lika and Northern Dalmatia). They settled in Slavonia, along with Czechs, Slovaks and some other ethnic groups, but not in Krajina.
I didn't said that Serbs were 100% in Krajina alone, but 70-80%, and what is important there were few of villages were Serbs and Croats lived together. Most setllements were monoethnic. Serbs lived in their villages, Croats lived in their own villages.
This map of part of Lika show that. Red are Serbs, blue are Croats.
https://i.imgur.com/5JAeDqp.jpg
This situation, along with religious difference, is fact that there were very low rate of mixing and intermarriage.