A lot of the boutique high-frequency trading(HFT) firms on Wall St don't have many or even any traders at all. Everything is traded via algorithms. These guys use the firm's own money to trade, no external customers, all private companies and all very secretive.

Here are some of such firms:

Getco
Wolverine Trading Company
Hudson River Trading
Chicago Trading Company
Jump Trading LLC
Optiver
DRW Trading Group
Spot Trading LLC

If HFT is what you are interested in, I suggest you brush up on your mathematics and computer science skills and focus less on economics/business.