You couldn't do it with first aid because that is a secondary skill. You can have as many of those as you want.
The secondary skills are... First Aid, Cooking, and Fishing.
The normal proffessions are, Alchemy, Blacksmithing, Enchanting, Engineering, Herbalism, Leatherworking, Mining, Skinning, and Tailoring. And you can only have two of them. Also, you can unlearn a proffession any time you want, however many times you want, but, lets say you get skinning to 150, and you unlearn it for another skill, but you don't like that so you learn skinning again, it will be at level 1. >_<
Edited, Sat Dec 11 13:27:55 2004 by GrandMasterDaveo