two questions

redef skip_services += {

You need to put these in both skip_services and skip_outbound_services.

Yeah, I know, this isn't intuitive :-(. The configuration for scan.bro
is pretty much a mess, and we have a rewrite of it pending, but haven't
managed to get it fully together yet. Sorry about that ...

2) How does the script choose the entries to be written
in the Scan section of the report? Reading the manual I see that they
should be ONLY the successful scans, but in the end of alarm.log file I
have some entries like "ScanSummary: host x has scanned a total of 3241
hosts" and this does not appear in the report! Instead, in the report I
have entries like "host y has scanned 100 hosts" so it's a lower value
and seems related to the thresholds set in the variable
"report_outbound_peer_scan" rather than being a total number of hosts

Right, the summary is decoupled here. Jason or Roger will need to chime
in here, as they're the ones who develop/maintain

Again, sorry about the confusion ...