Feingold isn't a progressive in the same vein as Kucinich. He's center-left at most, and he's had to fight to hold on in every one of his elections -that kind of thing tends to make you a centrist.

He's generally liberal on civil liberties, supported campaign finance reform, the stimulus, and healthcare reform, but not much else. Very much a part of the mainstream.

tl;dr Feingold/Kucinich is an incompatibility in terms. Center-left vs. far left.