Sorry for the delay responding, been busy on an important project.
Just installed the Widget CSS Classes Plugin (left the options under “Settings” >> “Widget CSS Classes” at their defaults) on my localhost test site, went to the Widgets area and widgets now have a new form called “CSS Classes:”.
I added a new class to the form on a widget (called it testing), saved it.
Loaded the site in a browser, viewed source (HTML source) and searched for the new class.
Found it with other new classes:
class=”widget-2 widget-even testing widget-container widget_text”
widget-2 and widget-even are added by the Widget CSS Classes Plugin.
I also see other classes associated with the other widgets like widget-3, widget-4 etc…
Looks like the plugins are compatible?
View HTML Source of your site and check classes like the ones above. To view source in Firefox and Chrome: “Right Click” the webpage (avoid clicking over images), “View Page Source”.
Use the search facilities to look for your classes.
David