Just tell when you upload theme that it was a collection of other peoples icons that you used and some you created (if you did). As long as you let people know that some or all were used from other themes you will not have a problem. If you look under the themes section in downloads there are some that people have listed that it was a collection of other peoples icons, wallpapers and docks. Of course some people created everything in the theme and they will tell you that to. but if you list that you created the theme and someone sees some of there work in the theme you uploaded that would not be right and someone will be very upset and will report you.