That is not-decided yet. Not sure which is the better choice. Shared opacity/size settings between those creation tools is the current behavior. But if in new design, these two widgets will be placed into their own ToolOption Panel, it is logical to have non-shared implementation as Blend Method does.
The Fill/Outline Color and Gradient widgets would be placed in Color Palette Panel. I have some other ideas to improve Color Palette Panel. For example, We can have a system default color palette which contains color chars such as tango palette, and a color palette which contains all colors used in the current project that user is working on it. Each color/gradient has its name, it can be called by type it in color/gradient parameter field of Parameter Panel. When we edit/change a color/gradient in Color Palette, all parameters linked to this color/gradient will be updated as well. Of course, the project color palette can add as many new colors/gradients as use expected, but we will also provide a button, saying “Cleanup”, to delete all unused in the project.
It is good to have more discussion since I am not using Synfig Studio in a professional way as most artist. 
How about placing it to Time Track Panel (and Keyframe Panel)? Currently, The Default Interpolation widget in Toolbox is for waypoint only, it doesn’t affect keyframe generation.