You said you didn't mind if the project had two different aspect ratios. If you don't mind the so-called "pillarboxing" that results when you put a 4:3 image into a wide 16:9 frame -- ie. black bars on left and right -- then there isn't much you need to do.
You can also help alleviate the pillarbox look -- which some people hate -- if you make it look like a deliberate choice. For example, shrink the still so that you also have black space on top and bottom, or fill any black space with a graphical element. Whether that works or not is, of course, dependent on the still and what you want to do. If you have information on the stills you need to convey, then shrinking them may make the information harder to see.
www.derekmok.com