One feature that would be very handy would be optional pop-up menus on buttons. So, for example, you would have one button that could have inserters. Click once to place the last used insert. Click and hold briefly to get a pop-up that has the other inserters (or whatever other items you have added to that button). This would allow you to greatly reduce the screen space required for toolbars, while still giving access to as many buttons as you need.

As for the UI, make it exactly like you already have. Just automatically leave a blank square at the end of each pop up, you click in that square to add a new item, which also adds another blank square.

You wouldn't even need to make this a different type button... Just add the behavior as the default, but since it functions as a normal button unless you click and hold, a user just using it as a single-item button would not have to change their usage at all.