Is it safe to mount 2.5" SSD in Mac Pro w/o bracket?
If you plan on "plugging in straight into the socket" that would be a very bad idea. The SSD is light, but all of that weight will now be supported entirely by the two SATA sockets on the computer and drive. Over time the small amount of weight on the SSD will brake the bracket on the Mac Pro (expensive), or worse the drive (data loss).

If you can find the correct cable you could run a cable from the socket to the hard drive, and double stick tape the drive like the above answer said.

Or just buy a 3.5 to 2.5 adapter and install it in the drive carrier. It is cheaper than repair.