The Skyview system itself only uses one GPS source at a time, no matter how many other GPS sources are connected to the other serial ports. Multiple GPS sources do not increase accuracy. At least one additional GPS source is installed by many Skyview users to increase reliability, what it appears you call redundancy. Backups allow Skyview to continue to perform to its full potential if for any reason the designated primary GPS source connected to Serial port 5 fails to provide the system with a usable signal. Skyview cannot perform many of its functions like mapping, ground track, ground speed, and wind readouts without a GPS signal available so a backup signal source is important to many users.
All GPS sources are connected through the serial ports. In multiple screen systems, any serial ports that have a GPS source connected are connected in parallel. Port 5 is, as you noted, is the preferred port for the GPS source that is designated by you in setup as the primary source. Additional backup GPS sources are connected to your choice of the remaining 4 serial ports. The backup GPS to be used by Skyview if the primary GPS on port 5 fails for any reason is specified by you in setup.
Skyview itself does not care if the GPS unit(s) are the 250 or the 2020. They are functionally interchangeable. The 2020 is only needed even in the USA only if you want your Skyview GPS to be the FAA approved position source for your Skyview transponder's ADSB out. Just as an example, even some Skyview users in the USA only have the -250 GPS connected to their system. They provide the needed FAA approved position for ADSB out to their transponder from their approved IFR GPS navigator.
