If anyone is curious, it is possible to invent/add a new prefix yourself for new tasks. I’ve done so in cases where I had a lot of data so I’m not sure how well it will work with smaller datasets. It’s unclear how well it transfers knowledge from the other tasks when you do this but my guess is it’s a lot better than starting from scratch. Parsing and creating basic representations of the input text is still helpful to achieving your task. Interestingly, it was still able to use the original prefixes and do translation etc. fairly well after training was completed on my large dataset containing only the new prefix.
The recommendation to reuse the summarization prefix is probably a good thing to try, it would be interesting to see results of reusing it vs not reusing it and adding a new prefix instead.