Most US equities exchanges only pay if you post resting orders (adding liquidity / market making). You pay a fee for removing liquidity (market orders). I think you are actually talking about internal matching at the broker here?
Risk management is far more complex. Risk management is more a part of the algorithm itself than a feature that we can build. That said, we can add more risk tools. We're very open to suggestions, if you have some in mind.
Eg: For risk management I might not allow any trading whatsoever when the VIX is over 40, and the 5 day stddev of the S&P is above some threshold.
Similarly, I might scale my capital usage based on my risk metrics. Or scale the capital available to a particular algorithm based on its individual risk profile.
Recreating risk management in each algorithm seems like a bad idea. But even worse is pushing off risk to the user to do in an ad-hoc way.
In the beginning, at least, it will be leveraged through your existing brokerage account. You're going to integrate Quantopian with your brokerage, and Quantopian will place orders for you with your brokerage.
If we're as successful as we hope to be that will mean we're driving a lot of trading volume. If you start driving enough trading volume, the exchanges start to pay you rather than the other way around. It would be a pretty sweet day if we can offer trading for free to our members and fund the company on the exchange fees.