In case you're wondering what I mean, I mean, should the brokerages only allow vendors to work or sell a particular specialty in order to spread out the support for figure s a little more?

By that I mean like they only do props, or morph's or textures. Specialty areas. 

Brokerages, being private organizations/businesses, should be free to restrict the products that they offer however they see fit. However, it would be somewhat unethical for a brokerage to forbid certain product types if they had not reserved that right in their vendor agreements, beforehand.  (For a stable business and a stable collection of contributing vendors, it's always best to dictate terms "up front", so that vendors know what sorts of products are acceptable.)

Just about every successful business manages its vendors just as well as it manages its products, sales and customers. It's a "rule..."

Brokerages, who rely on third-party products for their livelihood, that do not manage their suppliers appropriately are doomed to be forever at the mercy of their suppliers.

I bolded that for a reason - It's an important, indispensable, rule of businesses that are reliant on third-party products. NO business can dare to ignore this rule and still remain in control of itself. None. None of them... zero.. zilch... nada. Failure to manage your supply chain means that you will soon be in a soup-line and sleeping under a bridge! If you do not have control over your own business, since you don't manage your vendors well, then who has control over your business? There's only one correct answer.

To that effect: Incentives, marketing programs, product "pushes", seasonal promotions, collaborative efforts, these are all the hallmark of a well-run brokerage. And, if you have a well-run brokerage, you can leverage the cooperation of your vendors very, very, easily. Very easily. In some cases, the largest and most successful of these sorts of businesses actually have vendors paying them for the privilege of selling their products through them and participating in specialized marketing programs. (These are called "shelf fees" and "promotional fees", respectively.) I have worked with several multi-million dollar brokerages over the course of years of cooperative business arrangements and I know of what I speak. :)