Cluster Issues

Hey guys. Picked up an 06 GT that previous owner couldn’t get running. After some tinkering I have the car running and driving but I have a few questions.

First, the gauge cluster is inaccurate and kind of buggy. The inaccuracy is mainly the temp gauge and fuel gauge (both read low). The bugginess is in the tach (doesn’t move smooth, kind of jumps). The car came with a second cluster that works beautifully and is accurate, but the mileage is incorrect. Anyone encountered issues like that? Are they repairable? Could I add miles to the working cluster to match the current mileage?

The cluster brings up my second question. I thought clusters had to be matched to ecu and keys? The car will start, run, and drive with either cluster. The car only came with the physical key that you put in ignition and turn, no smart keys. I would like to eventually get a smart card/key if that’s possible and anyone knows the best way to get one.

Any help would be greatly appreciated!