Will the small size fit an adult that is 5’0” 100lbs?

There is a size chart on the 187 website. Pads typically fit very tight and inexperienced riders often say they are too tight. They are meant to be very tight so they will stay in place during movement or falls and are often uncomfortable for the first few uses. They will start to take the shape of your body as you continue to consistently wear them on the same body parts each time, meaning don't wear the left elbow pad upside down or on the right elbow one day and correctly the next. If you pay attention there is a right and a wrong way to wear them and the elbow is probably the most often mixed up out of all the pads. Also they will fit different when worn over long pants as opposed to when wearing them with shorts, or short sleeve vs long sleeves. Just expect them to be tough to put on and take off and a bit uncomfortable for the first while and you'll do better. My guess is that med/sm will work for your size but you'd want to check the site for possible fit suggestions first if you're not sure of your pad size. I'm about 165 lbs 5'10" and wear a med and love these pads. Happy to hear you're getting pads and not one of those too cool for pads or helmet types. You'll have a much longer skate life if you care less about other people's opinions and avoid the peer pressure. I knew someone who was very experienced and had a bad day that ended his life because he forgot to bring his helmet and borrowed one that he could not strap. Respect and love skateboarding and it will love you back.

The one I ordered was, but it will say free shipping if it is.

What' the difference between the elbow pads in this combo pack vs the elbow pads that are sold separately?

The only difference is the back of the pad. This pad is a slide on pad and the backing on the regular version is a neoprene and the backing on the Combo Pack version is elastic.