Even if your works are technically fair use under copyright law, even if they are transformative and separate from the original works, the IP owner can still sue you and the only way for you to maintain your claim to fair use is towinthe lawsuit. Tumblr will not protect you from lawsuits or contribute to legal fees. Do not take the bait.
Fellow fanfic writers, DO NOT DO THIS.
You 100% will get sued.
Despite what some people are saying, this is not a good idea. Abort the mission, abort the idea.
Younglings of Tumblr, hearken unto your elder, for mine aged eyes did see the days of Napster! People thought that buying a subscription to Napster’s music file sharing program meant they weren’t stealing the music they downloaded and the music companies did indeed sue them. So just because Tumblr is giving you its permission to monetize your fanfic (and technically they’re only giving permission to monetize fanfic that’s parody or other fair use), it cannot give you the permission of the copyright owner.