I ran some more tests today, and I realized that
my suggestion of using a SET operation was incorrect. If I have an account with 5
CalloutFeedItems and I issue a
SET operation containing only 2 new
CalloutFeedItems, then afterwards the only items mapped to the account for callouts will be the 2 new ones. What threw me off was that the original 5
FeedItems will still exist, but they will
not be used for callouts (won't be part of the
CustomerFeed.matchingFunction for the callouts placeholder type).
Sorry for the mistake on my part. The good news is that the guide you mentioned is correct.
I'm still waiting on more information from the feeds team on the ordering issue you raised, but I'll post back here as soon as I have an update.