when you first start modeling make sure your units are the same cutom units setup the first one is the units and you see the box where it says display units, same sure these are the same before you merge one scene from another,
you can always use the scale tool but your scene will look off kilter (not correct) so imho never scale your scene match the units and display units.
your scene looks dark b/c you have added a light and you have not changed near or far attenuation.
Attenuation settings cause distant objects to be dimmer. Attenuation is the effect of light diminishing over distance. In 3ds Max, you can set attenuation values explicitly. The effects can vary from real-world lights, giving you more direct control over how lights fade in or fade out
https://knowledge.autodesk.com/support/3ds-max/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/3DSMa...