in a previous post where i went over the blue mini jouster and gold mini jouster pets, i mentioned that there was no prerequisite in order to start the chain for the companions. however, mount hyjal has been refined and the quests have been updated since then.
you can no longer simply pick up the quests at the shrine of aviana upon entering mount hyjal. instead, you must complete the initial quest chain that's provided to you when you first discover the zone. after completing all quests up to the quest The Voice of Goldrinn (note: you don't actually have to complete this one; if you can pick it up then you are ready to head over to start the jouster chain), the quests at the shrine of aviana will unlock and you'll be able to start working on obtaining a mini jouster.
this information has been updated on the giant cataclysm pet list.
coming soon: will the blue and gold mini jousters fight each other?! are they still engaging in combat with random mobs?! once i finish up this quest chain, i'll have more information on these things. :)
**UPDATE: unfortunately, the gold and blue mini jousters still do not fight each other. also, blizzard has fixed the bug where the pets thought they were players. they no longer engage mobs nor are they flagged for pvp. however, they are still level 80 which is a pretty high level for a non-combat companion pet lol.