Oh man your right I didn't think of it that way = / hmm maybe instead of those sorts of ranks maybe Kasrkin...but there you would seem to be right there too there's not too many of them either.
Let's have a think... I doubt they would do it as so far it seems they're afraid to go too far from traditional, but what if the guardsman had a chance against a chaos marine supposing that first off he is well out of melee range and then he has cover.
My personal favorite idea would be he's busy fighting a nob(not happening with two factions sadface)and then you run up behind him jump at least half his height and gouge his eyes out with your knife.(then run away from the nob naturally)

If they fail Astrates be called; Astrates Imperial Navy Seals, they are not the majority of the army, they are special action, they do not hold each line of the entire galaxy ...

Ah, now if the Guard is being pushed back then more Guard Regiments & formations are thrust into the conflict. The Adeptus Astartes has it's own job to perform, which is to disrupt the enemies fighting capacity away from the battlefield - destroying facilities or supply lines, 'taking out' commanders or elite units, disrupting reinforcements - they rarely fight on the very front lines with the Guard. It happens, but is not common. Most of the time wars are won or lost solely by the Guard, without any Marines in the sub-system at all!

And, to be technical. since the Astartes do not fall into the Imperial chain of command at any level, one could argue that they are more allies than actual Imperial agents. If the High Lords of Tera want a planet taken, they order whatever regiments they want to the front. If they need marines, they ask nicely.

I suppose if the Emperor regained the powers of speach, then he could order around the lot. But since that is not the case, you could say that the IG and planetary defense forces are the only Imperial troops. Happily, the marines have similar goals!