nice deviantart OC as the new npc
Skeletron can be summoned via the Clothier Voodoo Doll. Sounds like the dungeon guardian however might not require killing it to fill in its data. It's also not impossible to get crimson and corruption in the same bestiary when you can import seeds from one world to another and create a new crimson or corruption.
EDIT: Worth saying I'm really excited for the bestiary feature. One of the things I was looking forward to most. I wasn't expecting an NPC to be tied to it.
However, it will be impossible to have Salamanders, Crawdads and Giant Shellys on the same world.
edit: also also, assuming the bestiary groups NPCs together that means there are at least 3 new NPCs we haven't seen at all yet