E-Mail Router installation planning

Hi,

We have a scenario where more than 1000 user mailboxes needs to be configured on CRM 2013 E-Mail Router. We are installing Email Router on multiple computers in a Microsoft cluster to provide high availability and failover. What are the best practices around the following?

1. Should E-Mail Router be installed on the server running the back-end roles or should it be installed on the SQL server box?

2. Any gotchas around managing 1000+ mailboxes? Is there is a better way of splitting these mailboxes into two instances of E-Mail Router?

Thanks,

Joe