Yes, definitely the wrong forum. RDS is not Postgres, and "parameter groups" is an Amazon thing. A quick web search would seem to indicate that the default group is replaced by the custom one, rather than enhancing it. But really, this is a simple "try it and see" sort of task.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/parameter-groups-overview.html Other than that, reach out to Amazon support. Cheers, Greg
