Scrolls and stones are listed as an oddity in the rulebook; they are things that are created by the mage prior to battle, so you are free to have as many dispell scrolls (if you're a beardy git) and powerstones (if you're an insane miscaster) as you wish.
Other than that, all magic items are restricted to 1 per 3 armies.
Also:
Amazing Aim adds +1 to a units BS. Should there be two heroes with this skill in the unit, it would be +2, etc...
No, you cannot take Lightning Reflexes more than once to affect multiple turns in a game. The rule reads that once per game a character may reroll all failed AS rolls.
Yes, you can take multiple WS or Ld Veteran skills to increase your statline to the maximum provided. Do I forsee that being a problem? Not really; you need to gain 3 victories to gain enough VP to give one character a veteran skill. To increase a Skaven's Ld to match that of an Elf would take several victories and allocating all of the VP to one hero from one list; should he perish in a duel, you've lost all your eggs and the basket they were sitting in...
The Adept skill is only useable by a character that already generates Power/Dispel dice. If you don't generate any dice, then you cannot generate any additional dice.
You can only re-roll a dice once. If you've got the skill & your opponent has the skill, then they will cancel each other out (note that BOTH dice are re-rolled when using this skill, not just your own). If you've two characters who have the Great Strategist skill & your opponent only has one, you can force a re-roll if you choose to.
Tom @ Red Claw

Do not question our wisdom, for in it is your doom.
Wood Elves: 2-1-4
Brahm, Disciple of Khaine, Lvl 17
Greantoof, Shaman, Lvl 8