Sentry prevents 1 damage per turn, and only from spells or abilities (note that Sparkshot and Overpower count as Abilities for this purpose)
Stealing the opponent’s thing does not trigger Arrives: abilities. (you can’t assimilate a Hive, since it’s a Unit, but you could Mind Control or Kidnap it)
Limit: X does not count tokens controlled by other players. see here:http://codexcarddb.com/ruling/limit_x. Note that this means if you steal an opponent’s Hive, and they play a second one, the second Hive’s Arrives: make 5 stingers will only make enough stingers to bring their total up to 5, per the Limit: 5 stingers/hive requirement.