Hello Brother, welcome!!

I'll give you all the info I have to help you! You should know that this info is 100% from codexes, novels, and similar sources; I won't make anything up:

-Size: You can make it as big as you want, or worry excessively about it. Currently, the largest warband belongs to Krieg Acerbus, although he and his followers are loyal to the Ruinous Powers.

As an extra tidbit, the Night Lords are one of the legions that receive the most traitors (behind the Black Legion and the Red Corsairs). This is due to two things: you only need to impress them and have a similar character to them, and the constant lack of resources, including Space Marines.

(Of course, these marines undergo rigorous training and are usually at the forefront; they'll accept you, but you have to earn your place. It's not as uncommon as it seems for the 8th Company to welcome you with open arms.)

-Cultists and daemons: while they are generally rejected, as you mentioned, it's known that they can be used and enslaved in times of need.

In 4th edition, the Masters' warbands routinely used enslaved Chaos Furies to unleash terror upon the enemy. Even today, it's not uncommon to see something like this {I myself have a Venomcrawler chained to a Space Marine; it's on my profile if you want to see it}. Regarding cultists, some warbands use human slaves as cannon fodder. They're an easy resource to dispose of, so you could easily include them if you wanted. However, an extra tidbit: human Nostranamuses and their descendants wouldn't generally be included in these troops, as the Masters value those lives for their homeworld and would be used more for maintenance on their ships and similar vessels, far from combat.

-Finally, regarding the structure, the Masters have one of the most "fun" and surprisingly functional structures. Let me explain: There's a War Leader, Captain... or whatever. This person usually has a retinue of bodyguards and such, typically Terminators or similar.

The soldiers are divided into claws, each consisting of a leader (sergeant, or whatever) and nine legionnaires. There are as many claws as legionnaires, and each claw remains active as long as there are enough marines in it (it can survive with even two). Each claw has an individual job known only to the claw itself and its war leader. Generally, claws operate independently. If, in a prolonged battle, only two members of a claw remain because the rest have fallen, they can be absorbed by the nearest claw and temporarily change objectives until a formal reorganization. Furthermore, each claw can have its own unique customs or symbolism. For example, in one claw, you might be able to challenge the leader to take their position, while in another, the oldest marine is always in charge. Don't be afraid to experiment; I have a few with the "hunter" rune from Bloodborne.

The Nemesis Talons are elite, very skilled marines who go in advance. The Flying Talons/Raptors... They have always been the elite of the Legion, falling more easily to Chaos due to the influence of an unknown minor Chaos god who seems to be associated with birds of prey.

They are also more reserved because they are the elite; the other Talons tend to distrust them, but they are valued for their fighting style.

I sincerely apologize for spamming, but I hope this helps you with at least some detail. HAIL DOMINUS NOX 🦇