As Imperial Commander, Helmawr would be responsible for the world's defense (PDF) and the cream of that crop would be tithed to the off world Imperial Guard. In older RT fluff, 'The Spiders' were the largest gang, Helmawr's personal police and the PDF all wrapped into one. Things were pretty loose at that point, obviously. When Necromunda the game came out the Spiders as a gang per se aren't mentioned, as I recall.
Then GW released 'Enforcers' miniatures which are Necromunda's police for lack of a better term, yet they're 'not quite Arbites'. I would say they're the new version of the old RT 'Spiders', drawn from all the gangs and fighters of Necromunda.
The way I see it, you'd have the following on Necromunda:
Arbites (Outside and above Necromunda hierarchy, guardians of Imperial Law)
PDF (Enforcers? Helmawr's personal troops?)
Noble Spire House Guards (paramilitary/militia)
Noble's personal guard (bodyguards/militia)
Lower Hive House Guards (paramilitary/militia)
Guilder/Merchant guards (bodyguards/militia)
Underhive Gangs, affliated with Lower Hive Houses
Outlaw Underhive Gangs
Scavvies/Waste Nomads/Prospectors/etc.
I made a noble's personal guard, but he's not painted. I'll try to get a pic. I use Void VASA figures for House Guards, which I see as people drawn from gangs and trained a bit and given better equipment.
This is all my personal take BTW... the various Necromunda novels are so disappointingly inconsistent its amazing so trying to get a 'canon' picture of what the Hive is like is fruitless.
ETA: There were some 'Helmawr' guards made when White Dwarf was spilling information about Confrontation. They were made right before the Necromunda box came out, I think limited release only, and didn't make it into the game. The 'Collecting Citadel Miniatures' site displayed all the figures ever made for the Necromunda setting... can't get it to come up at the moment.