You do NOT need to set the ICAO ID, unless you carry a Mode S transponder (where it helps to suppress your own transponder signal).
So yes, PowerFLARM WILL work 'straight out of the box'.
Do not set the *wrong* ICAO ID.... for obvious reasons.
Setting aircraft type ('Glider' for you guys and gals) correctly will ensure that the alarm algorithms treat you properly. If you configure as 'helicopter' and circle in a gaggle of gliders you and all others will get many annoying warnings.
It's a feature, not a bug... ;-)
PowerFLARM.us has sample configuration files which are easy to adapt for your taste.

As for setting the range to maximum as factory default:
We have many users who fly in very dense airspace. The default ranges ensure that they are only warned about aircraft that may become dangerous and there is no unnecessary distraction.