There is not central mattermost instance, so check with your instance admin if third party clients are OK with them. They should be, as the mattermost API is meant to be used for all sorts of funny things.
Your contacts’ puppet JIDs are of the form
john.doe is their mattermost usernames.
Your roster is filled on startup with the users you interacted with, eg, have a
“direct message channel” with.
There are no ‘contact has read’ markers in mattermost, so don’t expect them on the XMPP side.