Any pipe will work, you just will not be getting the maximum power out of the motor. You are in a sense choking it down. Same thing goes with your carbs. If you are running smaller carbs you are not getting the max power out of the motor.
Do you plan on having the motor ported at any point? If so that builder will more then likely have a certain pipe that he runs with his port job. Some like small bores, some like big bores, some like CPI, some like Shearer. Talk to your builder before you buy anything and make sure you are complimenting his work with the correct pipes and carbs.