To add to rick-slo's comment, one really doesn't need a lot of raw strength to play guitar.
There are a bunch of small muscles/tendons that work together for just about every motion you would want to make. So the isometric approach of isolation does not help, since you want the various muscles to progress together. I think the best way to build strength is to (referencing Frank Zappa) 'shut up and play the guitar.'
For flexibility/stretches, I have found a few yoga moves to be helpful, one in particular is called bound fingers or bound hands. It stretches the finger joints as well as the palm tendons. But the best finger stretch exercise I know of is a challenging tune or three with lots of movement.