frat and sorority pairings?

I would say:

Pi-Kapp: Chi-O, ADPI and Zeta (in that order)
Sigma Chi: Zeta, Chi-O, and ADPI (in that order)
ATO: ADPI, Tri Delt (in that order)
Sigma Nu: Zeta
Lambda: AOPI

It's kind of hard to generalize here. Pi-Kapp and Sigma Chi like to mix it up with the top three sororities because they can. The difference is that Sigma Chi and Zeta are really tight and Pi Kapp and Chi O are really tight. But then again, you have a group of Chi O's that only hang out with Sigma Chi's. It's honestly all over the board. Of Sigma Chi's six sweethearts, three are Zeta, two are Chi-O, and one is an ADPI. ATO is mainly ADPI with some Tri Delt. Sigma Nu is usually Zeta - you rarely see Chi O's and ADPI's going to Snu functions. Lambda Chi is usually AOPI. They don't do much. As for Phi Mu, they don't really pair up with anyone in terms of personality so it's really hard to say.

AOII and ATO