I think that Clemenza took over the Corleone family operations in the east. So, in that sense he formed his own family. But, he was probably still pretty much at Michael's command. Remember, in GFIII Michael tells Vincent that Zasa owns what used to be the Corleone family. Obviously, that indicates that there has been some transfer of power that probably goes back to Clemenza. Howeverm Mike is still behind the scenes, as Vincent says, like the Supreme Court.

And by the way, noone forms "there" own family. One forms "their" own family. Madonne!