I would suggest Capri's and a pair of "light weight jeans" with tank tops, ect., especially for the Disco, would be best, especially if you like to have a great time, party and dance. As for the weather, like most Carribean Islands, it does get humid at times, especially after if it rains.
I've been In June and October, and never did require a sweater any night, stayed both times in bld #13, right on the beach, was a bit breezy, but warm. One sweater should be fine, if you wear it at all, from what I know of the DR, there's only one type of weather, and that's summer! You notice in the each room there is a large umbrella, just in case if it rains, cause when it does, wow.......it pours, usually for 15-20 minutes.
Skirts and dresses are fine, as you will find, for dinner, majority of the tourist dress up. You can always go back to your room to jump into your jeans/capri's. What you would take with you would be the same type of clothes you would wear in the US or Canada during our summer months. Unless you are doing some of the tours, you will spend most of your time in a swimsuit. Have fun, let me know what you think of Flamenco when you get back.
Outside of Flamenco, there's the Plaza Bavaro, very safe to go to,all kinds of shopping, banks, rent a car, ect. Accross the street are many shopping huts. You will be hustled by the Dominicans to shop in their shops. A few Spanish words, will go a long way here. There is a couple of disco's in the plaza, a few locals, not a lot of people. A couple of sit down joints for a beer. The disco at Flamenco is great. RIU Bamboo's disco is about the second best.