Yes, as long as you say that USD = NBT your fund amount will remain fixed. Its not your fund value though. If you start with 10 BTC buy side and 1000 NBT sell side, then this method will assure you that you will have 10 BTC and 1000 NBT/USD afterwards. The value of your BTC of course changes.
BTC-E, or any other normal exchange, does not allow you to specify a price for an arbitrary large trade at a time, because it depends on the liquidity of their order book.
So let’s say the BTC-e price (average of ask and bid price) is 240 USD and you want to buy 10 BTC. How much will you have to pay? Certainly something around 2400 USD, but in fact its more, because maybe the bid price is an order which only offers 1 BTC and the following order has a higher price.
On Circle / Coinbase, these are promised fixed prices. If Circle says the BTC price right now is 240 USD, and I want to buy 10 BTC, then I have to pay 2400 USD (+ fee), in any case. This simplifies the calculation in the described order mirroring of the bots extremely, because the nubot only has to stick to the Circle price at any time and Circle promises you that you can exchange any amount at any time at this price (where any of course will be limited in reality, but probably out of scope for any LPC here).