Using the default mannequin, I get this shoulder problem using the starter pack animations.. Is there a fix for this?

It happens with the default third person animations as well.

I was able to make it better by changing all the bones retargeting to Skeleton instead of animation, but still doesn’t look right 100%

It is the way it is, on the animation starter pack by default. The gaps in the shoulders will fill up if you replace the mesh with a character that has full shoulders. This robot figure has these ball joints on the arms and the legs as well.
The first picture is the default mannequin and the second picture is with a different character:

The shoulder bone position of the mannequin is atrocious. Period.
And the animations epic gives are samples.
Nothing more. (Still, since 2015 they haven’t gasf to fixing any of it)

You can use my plugin to correct them - though you should work on the final mesh with the final rig, not on the mannequin.