I know this should most likely be in the "Dragonborn" section, but im kinda asking about the lore side of things more than the DLC itself, I'm roleplaying a Nord mage and I was wondering if it would be viable to join house Telvanni, has it happened in the past? Is it extremely uncommon or have races other than Dunmer joined house Telvanni in the past?

Other races can join House Telvanni, yes, and you can do so in Morrowind.