Wasn't sure if I may have missed anything in the guidelines, but aside from the rules on underage content, are there any other content restrictions or limits regarding taboo fantasy?
No, though we may have rules requiring tagging for content in the future.
US law is extremely liberal with regards to fiction. As long as it isn't a copyright violation or depicting a real (non-public) person's likeness, it's fair. The only reason the underaged rule exists is because we don't wish to cater to pedophiles.