SlideMagic only offers squares/rectangles as shapes. They are easy to align, and it is easy to fill them with text and/or images. From my 20 years of experience as a presentation designer, I have rarely had to rely on shapes other than rectangles to design beautiful slides.
I can see situations where more exotic shapes are required, for example in highly technical diagrams. For these instances, I would recommend making that diagram in specialised software, save the diagram as an image, or take a screen shot, and upload the image to SlideMagic. Make sure to set the colours in your diagram with the same RGB values that you used in SlideMagic to make the picture look consistent.