Okay, time for a history ramble!
This could be a long one, but hopefully it'll be worth it.
Alright! At the end of the Horus Heresy changes were made by Roboute Guilliman to ensure that no individual could ever again wield enough power to be a significant threat to the Imperium.
The Imperial Army was divided into the Imperial Navy (air and space) and the Astra Militarum (ground), the latter of which was further divided so that individual regiments were specialised. This ensured that no single Militarum commander had enough tactical flexibility to be truly dangerous, and that they also didn't have their own transport to move that hypothetical danger around.
The Legions of the Adeptus Astartes were broken down into Chapters, though most already used Chapters within their formation. A Chapter (assuming adherence to the newly-written Codex Astartes) would consist of approximately 1000 space marines divided into ten Companies of 100. In practice even an adherent Chapter may have more than this number however, as recruits in the Scout company often number more than their allotted 100 to mitigate the effects of casualties.
Prior to the Heresy it wasn't uncommon for Space Marine Legions and the Imperial Army to 'coexist', however this was also forbidden for the reason stated above.
Lufgt Huron of the Astral Claws rather famously combined his own Chapter with others, and drew on human elements in order to bolster the size of his forces with the aim of building a Legion. Feel free to look up the Badab War to read about the results!
So! As it stands a Chapter Master is limited in how strong his forces can be by the High Lords of Terra, but do they listen? Well, kind of.
On the face of things most chapters play by the rules, though sometimes the spirit if not the letter. Others however push the boundaries, and others still simply do as they please (usually with bad results, as above).
All Chapters have their warships and fortresses manned by Chapter Serfs, both for administrative duties and defense. These Serfs may simply be human, but they may also be Aspirants that failed the tests required to join the Chapters' Scout Company. Because they are required in defensive duties against enemy actions these serfs are trained and equipped to at least the standard of the Astra Militarum, though often to a much further degree. Some Chapters have been known to field their serfs in battle, which would definitely be pushing boundaries!
There is also the case of the Ultramarines. The 500 Worlds of Ultramar all maintain a Planetary Defense Force, which are answerable to the Regent of Ultramar, who in turn answers directly to the Ultramarines Chapter Master. Technically
the combined PDF are not part of the Ultramarines' military forces thanks to a little political obfuscation, but we all know the truth!
There are also some Chapters (usually those that retained their Legion name at the time of the Second Founding) that maintain questionably-close ties to their Successors. Feel free to read up on the likes of the Last Wall Protocol (Dorn/Imperial Fists) or the Unforgiven (Lion el'Jonson/Dark Angels).
Now, how does this affect you?
As we've discussed at least briefly, an Astartes Chapter cannot have a direct relationship with a Militarum regiment. The Dark Angels in particular are - as you know - incredibly secretive, and the idea of sharing in the Hunt would be unquestionable. Indeed, the Dark Angels have been known to leave the field of battle (often at huge loss of life to their allied human or Astartes forces) without notice in order to persue a potential Fallen lead. Having a Militarum regiment (and the required Imperial Navy contingent) would simply slow them down as they cannot mobilise as fast as the Astartes can, and in by Hunt speed is of the essence. Additionally, the Dark Angels operate on many fronts in small 'strike teams', so a single regiment would be of very limited use.
A Chapter does have serfs which are sometimes fielded in battle (usually defense), however to my knowledge the Dark Angels are not known to do this.
There is also the option of PDF, perhaps of a world the Dark Angels recruit from. While they typically recruit from feudal worlds (like Caliban was for the coming of the Lion and the Emperor) I'm sure that some are more modern, and would maintain a more modern military force.
Or you could simply say "they're DA Guard" - at the end of the day you can do whatever you want to, but be prepared for people to tell you you're 'wrong'!
Apologies if that was a bit long (and I hope it didn't drag), but if you want I can expand on any of the above. =]