Chie I think is a good match for Keiji because they both seem like fun-loving characters and Keiji is at least responsible enough to keep her on track with school work; he'd also appreciate having a snack dispenser nearby. Momo would also be a fun match for him because I can see him getting dragged along on her crazy conspiracy hunts. 

I'm not sure who I have in mind for Ryuuto, but it has to be somebody pretty serious that can put up with his occasional bad attitude.

EDIT: Actually, Kazuko looks like a good match for Ryuuto. They're both the "get down to business" type.