Not to dodge the request but most of your questions are really up to your personal tastes. The good thing is that finding those tastes can be rather... er... rewarding.
I wouldn't muddle the mint too much. You just want to bruise and abrade the leaves a bit, not mash them. Mashing crushes the stems and ribs which, in most varieties of mint, are bitter. The shaking will take care of the rest.
Recipes? Well, it's really just a tall sour. Start with the basic sour recipe of 3 parts spirit (rum), 2 parts sweet (sugar or simple), and 1 part sour (lime juice) and then go from there. It probably won't end up being your favorite, but it will be perfectly drinkable and a good starting point.
For a tall drink that's going to be served over a full glass of ice, I wouldn't worry too much about simple syrup vs. granulated sugar. Go simple and use the simple. Using the sugar to muddle is nice theater, but really doesn't make much difference in the end until you perfect your recipe.
Gold or light rum can often be less of a difference than where the rum comes from. For a tall summer rum sour on a hot day, I like a Cuban, Puerto Rican or Nicaraguan light. For something served up, more like a normal cocktail, I prefer gold.
And it's probably preaching to the choir, but the only other thing I'd recommend is to always, always, always use fresh, hand-squeezed lime. If you're not, then don't bother with any of the above and just get a jug of Bacardi and a pre-made mix.
Ooh, ooh, and try it without adding soda at least once.