It is best to disconnect the negitive terminal of the battery before doing any electronic work. Its just a good habit to do.
You will not need to have your instrument cluster flashed when you reconnect it. Just plug it back in and go.
You can replace the needles. Visit on of the numerous sites that sale replacement cluster faces. They have instructions on how to remove the needle. Basically, you can use some plastic spoons to pop out the needle.
If you note the placement of the needles prior to taking it apart and intalling it back in the correct location you will not need to do anything further.
*Tip: After you put the needles in place do not put on the front cover yet. Connect the cluster to your car and start it up. See if the gas/coolant/speed/rpm are correct. If not note the difference then pull the needle off and readjust. It makes it easier this way if you have to adjust.