You can get a device called "Safe Sync" to protect newer cameras from high trigger voltages of some older flashes. Here's a link to the one I have. http://www.adorama.com/FPHSS.html
The safe sync also keeps additional contacts of dedicated cameras and flashes of different brands from contacting each other. Only the center contact connects. However, you cannot get TTL flash unless the camera and flash are dedicated to each other.