Lots of people cut back their iris in summer, shortly after flowering and they grow new, cleaner summer foliage. And you aren't going to hurt stellas if you cut them back. We lift, bare root, cut back, and ship daylilies all summer and they bounce back just fine, so simply cutting them down now isn't going to hurt them
About your Iris, if there is too much mulch around them keeping them wetter than they want to be that might be creating some of the fungus issues. Only mulch lightly and try to let them dry out before watering the area.
Hope that helps,