You can add a block of IP addresses to the existing pool without deleting it. It is available under the "actions" tab. Yes they need to be on the same subnet as UCSM/FIs. If you want to use inband you can, but you would need to make sure that the VLAN you select is available on the data path of the FI.
You do not need both, it just depends on your network requirements. Most customers only use OOB. If you need both, yes, you need a new IP pool as it will be a different network for in band and out of band. The pool is typically different from UCSM subnet since its traversing the data network through the Fabric Interconnect uplinks vs the management network via the mgmt port.
Wes has addressed your inquiries. I wanted to add that one of the most common issues with the pool filling up is when ext-mgt-pool is used at the HW level (Equipment > Server > Inventory > CIMC > ext-mgt-pool assigned here) and then at the service profile level. Giving them two IPs, per every server. If you are doing this, simply remove the ip pool from your service profiles. This is non impactful (should always be done in maintenance window), as it is merely KVM access to the servers and doesn't require any reboots or make any other impacting changes.