Details and Conditions
From Frozenbyte's Contact page:
"We are fine with YouTube monetization using recorded video material from our games as long as the video is somehow related to the game(s) in question (e.g. review, preview, a Let’s Play, or some other similar type)."
"Streaming of our released games is also allowed (including Twitch and other services)."
"Also, it is allowed to use the music from the games in connection with videos or streams related to our games. However using our games’ music separately on videos or streams not related to our games is not allowed unless specific permission is requested (e.g. we need to see the video first or approve the stream concept beforehand)."
There are no explicit rules against streaming only for paid subscribers on Twitch. I would recommend contacting the developer for certain just in case as an extra precaution.
See more details by clicking the "source" link below.
What it Means
Videos and streams ARE allowed
You CAN monetize on YouTube and Twitch with ads and subscriptions
You MIGHT NOT be able to sell or charge for access to your content (this would include Twitch "subscriber only" streams or selling merchandise)
Any licensed IP or music not owned by the publisher are not covered by this policy