You can throttle down the output with a ball valve. Reducing the output by <25% won't harm anything. But really, after head pressure, it might be around 900gph anyhow. If you can return the pump for a smaller one, it's idea. Saves you money, electricity, and noise.
That said, the chiller will work fine with a bigger pump. It will just be less efficient. It is when you use too small of a pump the chilling chamber sometimes can freeze up to a block of ice 8)