The forwarders use an internal load balancing mechanism to determine the target for sending their data. The load balancing mechanism is not configurable.
Ideally, your indexers should all be identical -- same amount of memory; same number of CPUs; same amount of disk capacity for OS, splunk, hot/warm, and cold.
For best planning, based on those storage specifications, you should plan your indexes and retention for the lowest storage amount of 4.3TB, or a total cluster capacity of 21.5 TB (minus the needed storage for your replication factor). You are already seeing the impact of the inconsistent sizing with the errors noted in question 3.
It's not possible to determine if the retention of 1 year is problematic without the details of how your indexes are stored.
I would strongly recommend talking this over with 1) your Splunk Sales Engineer, 2) your local user group experts, or 3) Splunk Professional Services.
In the meantime, please read up on how Splunk clustering works here.
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