Re: Is there harm from having all the nodes in the seed list?
Well, having all nodes in the seed list does not compromise any correctness of gossip protocol. However there will be extra network traffic when nodes are starting because it will ping all nodes for topology discovery, AFAIK
See comments there for details on how this actually doesn't make any sense.
The "correct" solution is almost certainly to have a dynamic seed provider, which is why DSE and Priam both do that. But in practice it mostly doesn't matter except in the annoying yet common CASSANDRA-5836 case.