Found this on sale and was wondering if it would be better if I used a Y adapter (Startech 1 ft 2 usb ports to mini) for power or if the single is sufficient.
If they come with a Y cable, you can experiment using a normal USB cable to see if it still works (and if it does, then feel free to keep going). I once encountered a HDD that would not power without a Y cable (as the Y cable supplies extra power to the device). Y cables came to be during the USB1->USB2 era and were meant for plugging in USB2 devices to a USB1 host (which could not supply adequate power on a single port).
Nope. Some drives require AC Adapters because either (1) they were designed that way or (2) Y cable can't supply enough juice, so a wall plugin is necessary.
No, using a Y cable should not damage any devices.