Hi, I'm new to this site and have been riding bikes my whole life. I started building an old cannondale that has downtube shifters. My problem is that I don't have the shifters or derailed. I was told at my local bike shop that the shifters have to be the same make as the derailer?? Can someone explain to me what exactly I should buy,and where to look for the parts? Keep in mind the cannondale has a fat frame.

Thanks