I hope you put AI on first.
You chose Magery mastery. It will still do all the spells magery does and now it will do the mastery abilities.
It will now teleport when you don't want it to. There is something about Bless and Magery Mastery.
So no, you did not kill off any of it's magic skill.
Just make sure you get your cook to make blackrock stew, or he will not do his AoE, 1 stew every 24 hours. The stew is not too hard to make. If you have a friend who is a cook, he will help you if he has done the quest. The quest is not a big deal, just you have to know how to get to the place in abyss etc. You need the stew for the Lethal AoE and also bonding, and loyalty. Have fun with the pet. These are cool..
I suspect that most of them can make Chiv spec, but you will not get to 120 tact, they do if you go 115-110 tact, use Khaelor's tools and spend the training points carefully.
Sadly, people are paying gobs of gold for the "upper banes" because they think only those can make Chiv spec.
In reality I suspec ALL can make good, functional Chiv/Ai.
Also it is true that as soon as you put Chiv on these, the market for them shrinks, as NO PVP player would want it, just PVM. Your build is actually spanning all markets, including PVP. Nice, if you ever want to sell. And these fully trained and 120'd sell VERY extremely well. Much more than dogs fully 120'd.(Ive seen at least 2 times as much if not 3).
As Pawain said, nice if you put AI on it, possibly also Goo Aoe, though it will take 100 points extra, and these don't have too many points.
Magery mastery is actually the entire magery spectrum of spells, plus a few more frequent ones like teleport.
They Do bless themselves a lot, and if they cast negative spells with reflection and they are getting hammered then ride the pet and move on, wait for the spell to wear off etc.