@Shademeister  
It seems you didn’t quite understand what I meant. Yes shields are Shining Armor’s special talent just like gem finding is Rari’s special talent. Glimmer herself said she had practiced magic a long time to learn that cutie mark removal thing.  
Now what I mean is that there’s no limit in fan creations. For example a normal unicorn wouldn’t do much with a spell book if they really haven’t been training magic for a long time. Just think about how Twilight’s first teleportation spell was anything but successful despite having a huge amounts of magical powers and after being studuying under Celestia for years.  
There are many pictures even on this site where we can see normal unicorns casting spells way beyond their actual skills. Would you go to comment on those images that it’s not possible and it’s a bad choice to portray having much creater magical powers than they really have?
 
One could create a mage that is a unicorn, earth pony or a pagasus and why the hell not? There’s huge amounts of headcanons and fan creations that won’t fit in the canon world but it doesn’t mean that they are immediately a bad idea, something that shouldn’t be done. And yes, I am aware that some people seem to talk as if every single unicorn is Twilight Sparkle 2.0 and can outmagic canon Twi just like that. It makes no sense in canon point of view and it’s kinda annoying when some try to claim it as canon but if they treat it as pure fanon then it’s just fine. Let people have their fun.
 
I’m not that good with words but hopefully this made my point a bit more clear.
 
A little off-toic maybe but I don’t see unicorns running the high-society alone. As you menioned that I realized that I can name unicorns and earth ponies (Hoity Toity, Filthy Rich, Photo Finish, Sapphire Shores, Fancy Pants… actually I can name more earth ponies than unicorns) from this so called high-society but there seems to be a lack of pegasi there. Surely just a thing the animators and writers pay no attention to but still it’s clear it’s not a thing just for unicorns.

@Background Pony #413C  
What? Since when. Shining Armor has a huge shield spell, Rarity has her Gem finding Spell, Starlight Glimmer has a spell to remove cutiemarks as well as a shield spell of her own, Flim and Flam had magic to start up their machine, The night Guards in Canterlot have magic to cast light, we’ve heard of numerous spells being learned by Starswirl the bearded.
 
Just because we haven’t seen them casting off stupid spells all of the time doesn’t mean unicorns don’t have a variety of spells at their disposal. They can learn different spells they just have to practice them. They have set limitations between each race of pony. Earth Pony are masters of growing and manage to feed everyone with their skills, pegasi have been gifted with the ability to manipulate clouds and change the weather, and Unicorns are given the ability to cast spells and run most of high-society it seems. Each race is set with these limitations so that each of them have a place in the worlds lore. A purpose, if you pay attention to the shows themes. The entire premise of the show is about having a purpose. There being a place for everything. Regardless, I’ve supplied my counter point, and backed up my previous statement with evidence.

@Shademeister  
Well even among unicorns the only capable of various of spells would be the former unicorn Twilight Sparkle and Sunset Shimmer. Normal unicorns don’t go around casting too many spells either.  
So be it earth pony or unicorn (or even pegasus) it will go into the fanon area and there anyone can be pretty much anything.

@Background Pony #413C  
Because, last I checked Earth Ponies didn’t cast spells. It seems to me Mage would be a class best left to the obvious spellcasters, the unicorns.

Why would mage be a bad choice? I think it works just fine and that scroll thingy looks sweet.

@crazykhmr  
I dunno, I think Zebra’s fit more Shamanistic ways. Though I agree Mage is a bad choice. Perhaps more of a assassin type.

mage? why not shaman?