In synfig, when you create a region and a outline with the BLine tool they share the same bline by default. Look for the Bline List parameter on the outline or the region layer and see the “DYN” cyan letters close to the parameter name. It means that the parameter (in this case the list of blinepoints) is linked to a value node.
Unfortunately the ability to link the vertices you want, from an outline with different number of bllinepoints to a region (or other outline) is not an automatic task. You have to do it by hand one by one. Although it is not related to that task you can read the “Sewing BLines” tutorial and see how is the process.
Regarding to the character creation process you have to think on the character construction as part of the animation process. Some times is better to spend some time thinking on how will the character be animated and consequently construct the character in that way than build it directly form the drawing.
See this example I did and study the file. It might help you.
The fish character looks very good. Keep it alive! it has a lot of personality what it the 50% of the success!
-G