Under Etc->Options, a setting for more streamlined combo placement called "Advanced Combo Brush".

If you want to select a combo you have to right-click and select an option it from a drop-down menu.
If you want to draw a block of combos you have to right-click and select an option from a drop-down menu.
If you want to change the brush size you have to right-click and select an option from a drop-down menu.

Overall, it's a very tedious, clunky process. My proposed "Advanced Combo Brush" setting would be exactly like that in Tiled:

Right-click selects a combo.
Right-click dragging selects multiple combos.

If you want to go back to placing single combos, right-click a single combo. From experience, I find it to be a quick, easy system that enables you to build maps twice as quickly. It has no compatibility issues and the existing system could be kept as the default option. :)