On-the-fly custom POI

None of the garmins will let you do that, but what I do is I tap the screen and hit "save". Then, when I get home, I open the GPX file and copy out the saved coordinates and paste them into an existing one (if I already have one for that interest).

Is this a text format file, or do I need some software to decipher it?

GPX is an xml vocabulary. Not quite sure where I read that phrase...but it sounds good
It was designed to be the standard for transferring information between gps units and their software.

It's not designed for direct editing by humans (though that is sometimes possible...). Garmin's Mapsource reads and writes gpx format and POI Loader reads it. The units themselves do too (used when transferring Favourites/Tracks/Routes, depending on the unit).

Custom POIs that rival Travel Guides are possible, when POI Loader is fed with suitable .gpx input.

See signature for more info.

You could touch the screen at the location you want to save, save it as a favorite, then pull the info from the "current.gpx" file at a later date and create a separate custom poi file that way.

Or, download the redlight camera file here and delete the ones you don't want/need.

either way, a custom poi file will be needed to give the wanted alert...the download way is the minimum of fuss.

When creating a POI file, can I set a speed threshold for a speed camera? And, if so, will my 360 unit warn that I am speeding?

Do a search on "Red light camera alerts" as well as "Speed Alerts" and your answer will be answered in great detail, but in short, yes you can either by using the POILoader or the file name.