Library, training room, workshop etc. is requirement for a creature to join you but after that RGN takes place. So If you have only library you can only get shamans. If you have library, workshop, forge training room etc. Then your chances for getting shamans are smaller.
Size of a room does not increase the chances of getting creatures so having 3×3 library and 30×30 library has the same chances of getting shamans. However size of a room matters when it comes to unlocking different creatures. So For example for orc to join you you need 9 tiles of training room but for Ogre to join you you need 16 tiles. so having Training room of 16 tiles means that you can get either Ogre or Orcs joining your forces.
They have all different probability levels as far as I understand the game it works something like that game throw a die to determine if someone will be joining you or not based on your dungeon dormitory size etc. Then if outcome is YES games throws again to determine what creature will it be. Say you have 90% chance for Orcs and 10% chance of Ogre game throws the 100 sided dice and if you score 91-100 you get Ogre if 1-90 you get Orc.
Numbers are from thin air in this examples as I don't really know them that well but that how it works from my observation.
So Idea is on the very beginning of your dungeon decide what creatures do you want most and if you want to focus on strong military go for training room only until you get enough orcs then exapand. If you want a lot of production go for workshop only untill you have enough goblins etc.