I don't understand why you have "exchange to buy on" and "exchange to sell on". If you are arbitrage trader it does not matter where you buy and where you sell.
This is only for the first strategy available: "Arbitrage: Two exchanges, no transfer, one direction". It's a very basic strategy that will only watch one trade pair, and only buy on one exchange and sell on the other. More strategies are coming soon, now that the framework is in place it will be much less effort to add more strategies.
And how about "triangular arbitrage"
A triangular arbitrage strategy is already in development and will be available some time soon

We're also experimenting with "deep arbitrage" (inspecting multiple levels of the order book instead of just the top)
On all exchanges? I'm asking because all of them have different rate limits.
Rate limits won't come into play unless you're bot is attempting to submit multiple orders consecutively due to finding spreads very close to each other. All real-time data being used by the bot is done via public websocket connections, so the bot isn't making any REST calls until it's time to submit orders. Since most rate limits are for within a certain time period, i.e. X calls within 60 seconds, your bot would have to find a lot of spreads within 60 seconds to trigger a rate limit. This is still in beta though, part of the beta will involve testing these things and collecting data on how bots are working for our users. We'll see!