Sarkan is awesome, I also play him in my dragon EDH. However it can backfire pretty badly if you have all your dragons and then comes a board wipe. Also, something I also miss, thats imperative to have normally:
Karrthus, Tyrant of Jund. Giving all your dragons haste in this situation can most likely decide the game.
Also, if you really want to go full dragon: The Tarkir Dragons are pretty epic, especially the big five. Hey, all your dragons have double strike. Hey, all your dragons get +1 +0 for each attacking dragon. Hey, enemies can cast spells during your turn (Take that, wing shards)... aside of being pretty big and epic themselves.
Oh, and if you DO try to do a dragon commander
Tyrant's familiar is really funny to have. Especially with
Scion of the Ur-Dragon as commander. If you let scion become a copy of the familiar, he is his own commander, and the ability triggers
And concerning slivers: For a sliver deck, you NEED
Manaweft Sliver or
Gemhide Sliver. A
Chromatic Lantern is also one of the most useful things you can get for colorfixing. There are a lot of good cards for that, but for a sliver deck, without those you'll never have the right mana, especially if you go 5 colored.