As a general rule, there are a number of factors I consider before I even begin worrying about the next questions.
First, does the unit in question have a model? If not, can one easily be converted by the average hobbyist. If the answer is no to both, then I usually don't think it's a good idea to start including it in lists. There are some exceptions (such as the Storm Eagle and the Storm Talon), but as a rule I'll think it's a bad idea.
Second, does the unit desperately offend me through some combination of its rules, background, appearance or purpose? The Storm Raven and Storm Talon, for example, are both so ugly (and have weak enough background) that I'm less interested in including them in Epic.
Third, does the unit actually do something moderately interesting in Epic? If not, it's not worth including. For example, there are enough Russ and Land Raider variants that new ones are less compelling than they might be.
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1) editing existing lists to include newly released units by GW or Forgeworld
Depends on the unit and the list. The Space Marine lack of non-ally superheavies and flyers might lean toward the inclusion of new SM flyers or superheavies. Biel-Tan, on the other hand, are a pretty solid list as-is. As a rule, editing established stuff for shiny new money-grabs without models is a Bad Idea.
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2) adding newly released units to new developed lists
Fine, so long as it serves a purpose and isn't just for the shininess.
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3) adding units that exist in the background but without an official interpretation
I'm generally OK with it, but dislike it out of the suspicion that an official interpretation will eventually appear to ruin everything. Thus, I prefer its avoidance where possible.
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4) creating brand new units to fill out a newly developed list
As a rule, no. There's a lot of 40K background, and if you can't cobble together enough units for a list you're probably doing something wrong. Depends what you mean by "brand-new", of course.