Would it make sense to create a small storage volume for each droplet to use as swap?
Currently I am using a swap file on each droplet, which seems to be working fine, but I am wondering if having a separate volume might be better.
No, it would likely make performance worse. While block storage devices are connected to the physical hypervisor your droplet is hosted on via 40Gbps network links using the disks physically attached to the hypervisor will still be faster and provide better performance, for swap you are much better off using the local disk and using block storage for files.
