Sosa was pretty much as powerful, if not more so then some of the 5 Families. Anyone who wasn't a complete joke could get a meeting with Don Vito. The fact that he got Don Vito shot, was pretty much Fredo's fault for not having any balls and suspecting Paullie as the scumbag he was, and was really the spawn of Barzini or maybe Tattaglia.

No, Sosa was definately MUCH more powerful then Solozzo, and, you have to remember, Sosa ends up winning his war.