Can we do "anniversary-based" reviews? For instance, we want to do 90-days review for new hires, or review each person exactly one year after he is hired.
Unfortunately we don't support anniversary based reviews, or reviews based on hire-date in Small Improvements at this time. Instead, the timelines for reviews are defined by each review cycle. They're first created based on quarters, but you can customize the Timeline to fit your own schedule.
We'd suggest a workaround of creating between 4 (one per quarter) and 12 (one per month) review cycles per year, and adding new hires from a given time period to that period's respective cycle. While it's possible to create a separate cycle for each person being reviewed, it's not ideal. The fewer cycles you can use, the easier it will be to administrate from an HR perspective.
The option to have cycles setup by hire date is something often requested and on our product team's radar, but not something on our immediate or future roadmap at this time.