I was always under the impression that mage pets can use the shoulders and jewelry. At least I always gave my pets jewelry for the extra hp. Could be wrong on that though.
Summoned pets do have a limit on the amount of the gear they can hold, which is important to remember for at higher lvls. After a certain level (61? can't recall, haven't played the mage in a while) a mage pet comes pre-equipped with armor. If you try and give it a summoned set of armor, it will try to equip it and fill up its slots. Then, when you try to give it pet weapons, it will not equip them.
Not sure if other classes get pets at some point that have gear when summoned.
Ah, found this helpful post on eqsummoners.
http://eq1.eqsummoners.com/viewtopic.php?t=26262