2 Hints that I’ve seen on a few different cooking shows are, always do your vegetables on separate skewers, because veggies cook faster than meat and you don’t want to under cook your meats. Also, whatever you put on your skewers, make sure that there is a tiny bit of room between one chunk to the next, because the middle part of the chunks won’t cook evenly if they’re actually touching.
My cousin threw a kebab barbecue party and what she did was to put out each of the items (beef, chicken chunks, shrimp, vegetables) in separate plastic tupperware containers over ice (so everything would stay fresh and each item would not contaminate or flavor the other items by accident) then she had 3 different types of marinades in separate bowls with a brush for each one (one savory, one spicy and one citrusy). Then she had another large, longish container with her wooden skewers soaking in cold water. Then each guest would take a wet wooden skewer or two (this is so they don’t burn) and go down the line of meats and veggies and put whatever they want on their skewers, then they would brush on whatever marinade they wanted and then lay their skewers onto the barbecues. She actually had 2 big barbecue grills going, so there was plenty of room, and she designated the top section of both barbecues to veggies only (because she had, including myself, several vegetarians, so that was nice to be able to keep it separated) Everybody cooked their own skewers and took them off when they were done. It was really fun.
She also had mushrooms, cherry tomatoes and small sweet peppers.