I want to create a follower database that is on a smaller plan than its leader.
It is recommended that a follower be created using the same or higher plan than the leader database. Otherwise you may see performance problems as the follower struggles to keep up.
All production tiers (Standard, Premium, Private) have the same requirements. Followers can be up to 4 plans below their leader's plan, unless the follower's plan has burstable characteristics (*-0 and *-2 plans) in which case the follower can only be up to 1 plan below their leader's plan.