From the way they acted both before and after the meeting with Sollozzo, it's pretty clear that the Corleones expected a challenge after Vito said no. From the way everyone acted, it seems that the rules required the Tattaglias-Sollozzo group to bring their reasonable request, but that once it was refused by Vito, it was expected by everyone that the groups would test each other's strength.

I don't think it's a contradiction for Cuneo and Stracci to stay out of the Tattaglia-Corleone fight until after McCluskey was killed. The McCluskey killing was the incident that brought the authorities down on all the families.