Indeed, the keyword use is really interesting. Yes, you can make a Battle Forged Imperium army out of one Detachment containing Imperium units.
But the more faction keywords the units have together, the more synergy you get out of it. Very interesting I must say, very interesting.
As I plan to assemble my Guards army, I will see to that the main force share three keywords. Imperium, Astra militarium and "Regiment". I will add a unit of Militarum Tempestum who share Imperium and Astra Militarium but which have their own specific "regiment" keyword, Militarium Tempesus. Ordinary guards that only have "regiment" keyword cannot be given the keyword Militarium Tempestus.
The keyword regiment is replaced with a name of your chosing. If you choose Catachan or Cadian, you can use the synergies from special characters like Harken or Creed. But if you make your own regiment or field Steel Legion, you cannot change the keyword on Creed to be "Steel legion", for example.
Keyword is a really sweet little tool.
I will strive to make an army from the Battalion Detachment. I have three Infantry squads for troops, A Company Commander and a Lord Commisar for the mandatory choices.
I will add a Militarum Tempestus squad to the troop rooster. That leaves me with two more troops slots for the future.
I got six elite choices, but now things gets bit more tricky because command squads and platoon commanders takes up elite slots! I do not need them but this is the Guards! So one platoon commander to help out and a command squad to bring me a medic, an extra vox for my Commanders to use and a crack shot autocannon.
Then I have my veteran squad taking up a third elite choice.
So I have now only three elite slots left. But I think that having extra commanders will be useful in an army like Imperial Guard.
But playing a patrol detachment with Guards, those two elite slots will some really, really though choices to make.
As you see, using the Patrol detachment you only have two elite slots, that would be the command squad and the platoon commander, leaving me with no vets.
If you want to place cool limitations that will add to the challenge of a game, you can agree to use only a Patrol detachment for exampe, points or power levels. This will help keep sized down. However, you better use points or powerlevels two because a Space Marine patrol will be much more powerful than similar sized Astra Militarium one. As it should be.
I see the matched patrol detachment battles to become a nice way to get a reasonably sized yet fast game.
Maybe I should start a semi-blog here for my organization of my Astra Militarium army?