Aperture is not the best tool for making collages -- and may not be even an adequate tool. It's strengths are RAW conversion, image devolopment, image storage and retrieval, and image publishing. There are two sophisticated tools for publishing groups of images -- Slideshows and Books -- but other than that any time your work involves combining graphics elements, you should think first of a graphics editing program. Aperture is an image development and DAM program.
However, you may find Aperture's Light Tables readily meet your needs. You can combine, overlay, resize, and print groups of images. Worth knowing about -- Light Tables can be useful for many tasks.