Child-of: I do think it depends on what DAoT world the space marines would be attacking. If we're talking about a DAoT military installation, then the marines are screwed.
Exactly, this kind of hypothetical situation depends almost entirely on the circumstance.
Was mankind more or less united during the DAOT?
Well the Dark Age humans have been referred to as the 'Human Federations' I believe.
Generally speaking they were likely to have been very loosely unified. I imagine sectors and systems would have been extremley indepedent, but I think Terra would have had the prestige (and the finance, material and military force) and thus plausably the influence to enforce some kind of limited authority over many colonies.
That being said though they were no way near as unified as the Imperium.