@mskl is spot on. Paralax out of San Francisco is the poster child for this. They have become the third/fourth(depending on the day)largest MM acting as a professional customer. You'll need massive scale in technology and communications and when your all done you be tens of millions in infrastructure. Keep in mind 5 or 6 names are about half the exchange flow most days and they are probably equal size upstairs most days.
But some of the spreads, oh man, are just too tempting...