Yes one starting clamp there is enough. You will use one flying clamp for all the left side mains and the other starting clamp for all the right side mains.
Starting your crosses works the same. Clamp the top cross with anstarting clamp and pull the second cross. Then you have two strings to clamp on with the flying clamps.
When you pull tension on 1LM you will not have full tension on 1RM because of the friction at the frame connecting 1LM to 1RM. But when you remove the starting clamp after pulling tension on 1RM all will balance out. Same on the crosses. You will not have full tension on the top cross until you remove the starting clamp. I do that at the very end.
When you're dead you don't know you're dead. It's the same way when you're stupid.