Hello,
don't worry of not getting back to Cabarete from Sosua at night. There are public taxis at the taxi stand opposite la Bomba in el Batey district of Sosua. Trouble is, they usually wait till the car is full (6) to get going which can take hours after midnight. Solution is to buy the 6 places yourself, you will still be better off than paying a private taxi. I have done it several times between POP and Sosua. If you paid the public taxi on the way to Sosua, you can figure out the price for 6. The ride in a public taxi should not cost more than 15 pesos per person. If you are new to the area and don't know the price, never ask the taxidriver for it, just give him what you think it will cost (not a lot), if it is not enough, he will surely tell you, if it is, he will assume you are a regular visitor or a gringo who lives there.
Other options are private taxis or motoconchos but I don't recommend the last ones at night, it's not very safe and the distance is too far to be comfortable. Have fun!