Specifically, are there slang terms for the following, both of which might be used contemptuously by a dealer:
-a newbie drug user who doesn’t know what they’re getting into; a naive sucker.
-a user so addicted that they can be virtually enslaved by whoever will supply them.
These two terms are widely used to refer to people that are strongly addicted.
I’ve heard the term ‘fiender’, which is short for ‘dope fiend’ used to describe a person that will do or say nearly anything to acquire more dope.
I’ve also heard of female meth addicts being referred to as ‘bag hags’, with the implied meaning that the woman will do anything desired for a small bag of meth.
Not sure if this is quite you were looking for though. 
Sorry, no cites.
“Crackhead” comes to mind—there are probably other “—head” variations.
Tweakers, comes to mind.
skel which is short for skeleton since many of them are super thin
My brother’s supplier called him Toby, after the character in Roots who was kept chained.