1. A system, comprising: a plurality of hardware computing devices configured to implement a plurality of resource instances of a provider network; and one or more hardware computing devices configured to implement a resource manager, wherein the resource manager is configured to: in response to receiving, from a first client, a pool assignment request specifying a first pool identifier, assign a first one or more resource instance reservations to a first reserved instance pool, wherein the first pool identifier identifies the first reserved instance pool; create a link between the first reserved instance pool identified by the first pool identifier and a second reserved instance pool identified by a second pool identifier, wherein a second one or more resource instance reservations are assigned to the second reserved instance pool; and in response to receiving, from the first client, an instance activation request specifying the first pool identifier identifying the first reserved instance pool: identify a particular reserved instance pool from which to select a particular resource instance reservation for activation, wherein to identify the particular reserved instance pool, the resource manager is configured to: determine whether there are sufficient resource instances in the first reserved instance pool to satisfy the instance activation request; and select, based at least on a determination that there are insufficient resource instances in the first reserved instance pool to satisfy the instance activation request and on the link between the first reserved instance pool and the second reserved instance pool, the second reserved instance pool as the particular reserved instance pool from which to select the particular resource instance reservation for activation; and activate on one of the plurality of hardware computing devices and responsive to the instance activation request that specifies the first pool identifier and based at least in part on said select the second reserved instance pool as the particular reserved instance pool, a particular resource instance of the plurality of resource instances corresponding to the second one or more resource instance reservations of the second reserved instance pool.