I have an artist who draws for me and he sent a profile sheet of some different heads to use on the body's already created.
When I import to Toonboom animation the image files Are in large scale and when shrunk down the match the body the heads lines are thinner than the body's line thickness.
Is there a way to import the image or scale it down prior to importing so as to keep the same line thickness?
Nope. You will have to extract the lines separately using selections or whatever and then treat them with fiolters or layer styles to expand them - with all the possible downsides.