Well, if asking one human to enslave another seems strange to you, then what about asking a Seraphim to do it? I mean the Seraphims are considered to be noble warriors of light, and one would suppose a holy warrior should be irritated by such ignoble request. And yet the guy who offers this quest would offer it to everybody regardless of appearance. He probably just thinks that all wandering adventurers are rogue outcasts who only care about the money and that his money is as good as everybody else's. If you are playing a Light Path char, you quickly prove him wrong by demonstrating that some adventurers do put morals above the money. By the way, it seems to be a common attitude among Tyr Lysia questgivers - if you are playing the Dryad you even have some people openly assume that you are adventuring due to being unfit to stay home and treat you as a renegade.
As for the Dragon Mage - there are several characters who are openly called "Dragon Mages" in game. The playable character appears to be human (round ears, sturdy body and so on), and, as Flix said, is formally classified as such. Three other Dragon Mages he encounters during his quest (Gemblik, Fangil and nameless Dragon Mage Adept you find on the road) share the human appearance with the playable character, and the fourth (Varmin) is a definite High Elf. On the other hand these characters do not ever call themselves anything other than the Dragon Mages, and put a lot of value in being chosen and mentored by the Dragons. We are never told how the Dragons choose their apprentices, so it might be that there are Dragon Mages who belong to different races, and the different look of the guys encountered during the Dragon Mage's class quest seems to support this theory. From the journal entries and spoken comments, the playable Dragon Mage appears to be rather sarcastic and looking down on pretty much everybody except for Dragons. He is not kind toward humans, but his remarks on the High Elves he meets during his class quest are no better, and let us not start on goblinoids. So it appears that being a Dragon Mage is a cultural thing - their race is secondary if it matters at all.
The Dragon Cultists of the Dragon Sea Islands are Elves for sure, but they are never actually called Dragon Mages. They are described as "dragon worshipers" and appear to be the servants of the Dragons but it is never mentioned that the Dragons teach them anything or see much value in them at all. They never use much magic, preferring to attack with polearms. From the quest about the conspiracy within the cult we learn that the leaders of the cult are conning their subordinates and only want to rob them of their money, so they are probably charlatans and there might not be any real Dragon Magic behind the cult at all. Then again, some of the cultists - certainly not all of them - might be actual Dragon Mages. In the end it's up to the player whether or not to think of the Dragon cult and the Dragon Mages as related groups. The game leaves room for both opinions.
One more angle that might be worth considering is that there are two locations themed around Dragons and their followers - the Island in the Jade Sea where the playable character starts and the caves under the Dragon Sea Islands. In the first place you meet a couple of human-looking Dragon Mages, and the second one is populated by elven Dragon Cult. So it's possible that the southern Dragons choose their apprentices from some human tribe, and the northern Dragons from the elven cultists.
My personal opinion is that the playable Dragon Mage is technically (biologically) human, but he renounced his origin when he was chosen to be apprenticed with the Dragons. Now that his study is complete, he considers himself a superior being and would be offended if someone bundles him up with the commoners of Artamark who never had an honor to talk to a Dragon let alone study from one.
Then again, the Sacred 2 lore is a bunch of confusing and mutually contradicting texts - you can read rather different descriptions of the world, its races and events depending on which character you play and which path you follow - so every player can make everything he wishes out of it.