How do frats choose who they mix with

How do frats choose who they mix with. Hate to sound like a whining bastard, but the frats im interested in mix w srats I havent liked so far at parties. I thought it was j bc theyre older but I also get bad vibes from the girls they are dirty rushing... is it common to not like the srat u mix with? feel like id enjoy the brotherhood but would prefer the mixers of a lower frat more.