What sort of dangers exist in trying to run more than one pod on a single server? I’m thinking of thing such as them sharing a single Postgres installation, or being proxied by the same nginx instance, and so forth. Is there any issue with doing this? I know that it is possible to get a single pod to be run with multiple servers, but what about multiple pods off a single server?
As long as you separate your redis between the pods correctly (either select different databases or start multiple redis instances) there shouldn’t be a problem with that.
Different Redis databases as Benjamin says, but of course don’t forget to change the database name for at least one of them too 
Okay, so I need to change diaspora* instances in some way to ensure that they use a different Redis database name as well as a different PostgreSQL database name, is that right? Of course, each Diaspora* instance will also need its own uploads directory as well, right? I assume that they can share a proxy to assets/ because that will be the same between each instance.
If I am using the S3 interface, does this mean that my uploads/ directory is handled by S3?
It might be technically possible to share anything in the public folder but I would recommend against it, it’s much less complex to just give each installation their own.