I have a question it's a rookie question.. if I have an autometer water temp gauge and as soon as I turn the key on with the motor being Stone Cold the meter jumps to 200. So here lies my rookie question , do I need to have an autometer sending unit or if I put a different brand sending unit would that give me false readings??

If you disconnect the wire at the sender, does it read cold?

If you disconnect the wire at the sender, does it read cold?

yes

this is the meter I'm running

In order for the gauge to work properly you should be running this sending unit (ATM-2258).  You can find it at summit for about $16.00 + shipping