You are free to use any software, but our recommendations are listed in the Software Recommendations page.
Feel free to reach out to your Compositing Supervisor, the Technical Director, or make a post in the RotoKey discord page (tagging both your Supervisor and Technical Director)
TIFs are optimized for our specific workflow for the following reasons.
TIFs Provide an Alpha Channel
TIFs can have lossless compression to reduce file size
TIFs can be written as premultiplied (matted) vs PNGs that can only be written straight
TIFs are far faster to load and write as an image sequence then PNGs (up to x10 times faster)
TIFs are a more common format for use/viewing especially among less experienced Artists when compared to other professional formats such as EXR