I know there was a very recent change (last month or two maybe) to put in the Worker's Compound requirement to help balance Custom NPCs so you couldn't just pay gobs of CPs and spawn a brand new squad of whatever race you wanted. Within the last sync or 2 came the updated rules.
As far as I know, the process is: 1) Be in a Worker's Compound + submit the request(s); CP gets deducted at this point.
Then wait, and when the NPC is approved by the NPC team, it gets spawned at your location, wherever that is.
Of course they can join your party and fight. That is the reason the woker compound and 3 per week change came in. They took out te ability for custom npcs to fight because of how easy it was to spawn gobs of the ideal race right at your location. It was a very temp fix until the limit was placed to remove that exploit.