Can I choose the original UE4 Mannequin Skeleton when Importing the rigged character?

Yes you can.

Beware that the original UE4 Mannequin requires bone rolls that are incompatible with the way Blender 2.8 works. You will need to maintain two separate blend files. One to create animations and another to export with bone rolls that support import into the engine with the original UE4 mannequin skeleton.

A detailed video on this process is available on the youtube channel.