Hay!
1. To remove a npc, look at the npc you want to remove then do /npc sel. After do /npc remove
2. I'm pretty sure this video can help you with making an army https://youtu.be/tuyTAWVSjbE
3. I'm not to sure how to set the skins of the player that has joined with npcs sorry.
I hope this helped!
Hmm I think with passive mobs they need a certain block to spawn on. Like, if your trying to get a pig you'd need a 3x3 block of grass blocks around it. Have you tired
1000 forum posts = 1000 credits give away. Now I know you can't do that but what's something that you can spend with around 1000 credits... Ranks! So basically a rank giveaway/upgrade. Or just a face reveal would be more interesting