Shoot in the case of these starmines, it's little differ from case of sigle launching. Because, like any starmines are launching a lot of shells during a fixed time,it's offten overexposure. That is, Set the regular step value, do expose when launching fireworks, close it when as like as you, this is all about camera operation. Let check the one of photos that taken by some setting. You will learn a great deal from any photos that you taken.
Many hints are there.
You find that , More many shot was in to frame look more brilliant, more few shot looks briefly, over exposure or less ,etc. If you make the best use of these hints, you will be take more nice photos at next time. Anyhow please try to take your first fireworks photography. About multi exposure Most of fireworks photos are taken by multi exposure. There are some way to multi expose, but nothing there that this is the best way.