The other groups are best tackled individually. For example, the Ionians have no ferry connections to the rest of the Greek islands; the same story with the Sporades and the Argo-Saronics. The NE Aegean islands do have a link to the Dodecanese and the Cyclades but are probably best left for another trip once you have got the feel of the rhythm of the Greek islands.
Since airline reservation requests are often made by city-pair (such as "show me flights from Chicago to Düsseldorf"), an airline that can codeshare with another airline for a variety of routes might be able to be listed as indeed offering a Chicago–Düsseldorf flight. The passenger is advised however, that airline no. 1 operates the flight from say Chicago to Amsterdam, and airline no. 2 operates the continuing flight (on a different airplane, sometimes from another terminal) to Düsseldorf. Thus the primary rationale for code sharing is to expand one's service offerings in city-pair terms to increase sales.