Anything more than taking the general premise shouldn't be done without getting explicit permission from the creator. General ideas aren't things you can really lay claim to because they're so very broad and ephemeral, but things like settings and characters and their posts are clearly and firmly their work and should be treated as such.
Giving credit is required when using someone else's original work, but it's not when you're just using the general idea. I suppose it's the nice and polite thing to do though, if that means anything to you.