As mentioned, only carbonate water in a soda stream. Anything else will explode/erupt and make a mess. If you are going to try it anyway, use a very small amount of fluid (wine, for example) and only carbonate it with very short bursts (less than a second at a time).
RH, it's perfectly fine to use the syrup with normal water. You can also get official Pepsi syrup online (it isn't cheap however it will make the perfect flat Pepsi every time - if you go down this route you may need to try a few different types of water (tap, bottled, filtered)).