Cut the pork shoulder. Pat the pork shoulder dry with a paper towel, then place in the freezer for 20-30 minutes to firm up. This makes it easier to cut. When ready, place the pork on a cutting board, then use a very sharp knife to slice (or shave) the meat into ¼" thick slices. Don't worry about the length if some pieces shed, just focus on even thickness.
Make the marinade. In a large bowl, combine ½ soy sauce, ¼ cup rice vinegar, ¼ cup oyster sauce, ¼ cup brown sugar, 3 Thai chilis, 1 Tbsp fish sauce, 1 Tbsp ginger paste, 1 Tbsp Kosher salt, and ½ tsp black pepper, then whisk well.
Marinate the pork. Add sliced pork to the marinade, then press down to ensure all meat is fully submerged. Cover tightly with plastic wrap or a lid, then refrigerate for at least 3 hours, or up to 2 days.
Soak the skewers. If using bamboo skewers, be sure to soak them in a bowl of water for at least 30 minutes before assembling and grilling.
Assemble the pork skewers. Carefully thread a few pieces of pork onto each skewer, going down a little more than half way. If the slices of pork are wider than 1", cut them in half so the skewers are roughly uniform and cook evenly. No meat will go to waste, you can place even the smallest pieces on skewers!
Make the pineapple skewers. Take all the pineapple and assemble them onto their own skewers, going down the same length as the pork. It's okay for the fruit to be tightly packed.
Cook the skewers. Preheat a grill over medium-high heat. Make sure the grill grates are very clean, then coat with grill spray. Add skewers, leaving at least 1-2" between each. Cook covered for 3-5 minutes per side, or until fully cooked through, about 15 minutes. Pineapple skewers require only 4-6 minutes total.
Serve immediately. Allow to cool slightly, then serve immediately with extra Thai chilis, if desired.