1. First, you have to say how the SCI device is configured. Some have and din and a dout, other just a single I/O pin. So you need to provide more information. Perhaps tell us the part you wish to talk to, with a link to it's spec sheet.
2. Not enough information. In general there are no issues with stdio.h If you are expecting to open a file, you may have trouble with that, but things like sprintf work. Which functions are you having trouble with?