You have a lot of say as it is your money and you have to live with the outcome. The PB does provide his expertise in project planning, pool design and hiring subcontractors if he does not have in-house capabilities. He should provide you a plumbing plan that shows all the suction and return pipes plus a basic layout of the equipment pad. The plumber will have some freedom to make it all fit, but you can input what you want such as using diverter valves rather than ball valves, asking for a heater by-pass, removing the chlorinator, etc.