Well using one or two fonts are ok. You can always keep eye on how many fonts you want to use and Page speed using their tool. Go here : http://www.google.com/fonts and add fonts to collection. When you click on the USE button below. You’ll see the PageSpeed effect see this screenshot : http://prntscr.com/5v40we . Make sure they are always under Green level.
Regarding making 100% compatible, well you know the classes and id of widget can be different and different classes can be used using Widget CSS Classes plugin. So if you can figure out the selector of the element then you can use it everywhere inside the plugin. This video might help you : http://vimeo.com/77878709