I would not recommend deleting the NPC which come with the pack. You can always change the appearance to something which you more like.
If you delete these NPC, you just get generic answer if you ask this NPC about Moonwood Mill. If you delete the leader the game will not create a replacement for the packs which can causes issue if you want to try out werewolves and the packs mechanic. Same for Greg, which is a hidden household and will not get a replacement if you add him to your played sims / delete him. (Bug Report: https://answers.ea.com/t5/Bug-Reports/OPEN-W-Packs-don-t-repopulate-new-members-no-leader/td-p/11572646)
As @GiardiniDiMarzo answered; You can keep werewolves only in Moonwood Mill. You will not see other werewolves in werewolves form in other worlds (they will still visit in human form). Greg will not visit other worlds. The World is an occult world, so it has the same function as the magic realm. The game will create werewolves and these werewolves maybe visible if you live there.