Nofe

Well yes, if the creature is on a separate layer, you can resize him (or her) easily.
 
If you just click on the creatures layer, you should get a bounding box around the creature.
 
You can then drag on the corners of the box to resize it, independently of the other layers.
 
Here you can see just how bad my drawing is.

What you're likely seeing them do in the videos is selecting the part with the freehand selection tool, then using the move tool to manipulate it.  You'll quite often see them doing it with eyes for example to fine tune them to make them look better proportioned/orientated.  Obviously it helps to have organised line art layers.

 

To blend colors with mixer brush you have to lay down at least 1 in between color and clean the brush all the time