The fixed terminal issue is a minefield as it seems to be only defined by the states transport regulations. I vaguely remember a civil case where a pilot was sued for damages for injuries caused during landing. Aparently because the flight returned to the point of origin it did not travel between terminals and was deemed to not be a charter or RPT and was not covered by carriers liability act.
Charter can be between fixed terminals and fixed schedule as long as the seating is not individually available, mining charters, tour company booked flight for sightseeing etc...
If there is no fixed schedule then seating may be available on a charter, but this is where it gets very grey.
RPT can be done in any aircraft which complies with the rules, PA31 and C402/4 types have been doing it for years and continue to. In the USA PA31 RPT still occurs under commuter category rules. There are minimum performance requirements that these aircraft must meet. Whether the aircraft can actually achieve these figures is another debate and is still a valid thought for some much larger transport category aircraft I know.