You should be able to scan at the listed locations. Just make sure that your scans are done at a high resolution (300dpi), and saved in TIFF format. You want to scan at a high resolution so that you don't pixelate the artwork if you need to enlarge it. If your artwork is drawn smaller than you envision it you may want to enlarge it prior to scanning.
Different people print on different materials. Some have had success with Kinkos, but I see Mamemarquees mentioned in a lot of project threads, and believe that they use vinyl or a similar material for their CPO overlays. I haven't gotten to printing yet, so perhaps someone with more experience could chime in.
Photoshop documents should be readable by both PC and Mac versions, the only issue is if you have a newer version of PS, and are sending it to someone with an older version. I'm pretty sure that there are compatibility options when saving in PS to alleviate those kinds of issues. If you send your PS document to a PC user, you will want to make sure you add the file extention to the end of the file name .psd, .tif, .jpg, etc. so that the windows user can more easily open the file.