Do I need an additional booster if I run isolated sections through a PM42 or can I use one Zephyr? This is a small layout design (5X9) and looking to make it as simple and safe as possible.
This is really a question of how much power your railroad requires. You don't state what scale you are operating in, so a 5x9 in H0 might only have three or four locomotives, while a 5x9 in N-Scale might have fifteen or twenty locomotives.
Initially, you will be making an estimation based upon your power requirements and the output power of the 2.5 Amp Zephyr. If the Zephyr alone does not provide enough power for your needs, boosters can be added as required. Because the Zephyr has LocoNet, your expansion of power through additional boosters can be done easily, with LocoNet providing the data network for the railroad. You connect the booster and its power supply to LocoNet, making the required configuration necessary for the booster as needed.
Likewise, the PM42 makes this sort of expansion easy because the PM42 has four sections, each of which is isolated from the others. So, initially, you would connect the output of the Zephyr to one section of the PM42, and from that section of the PM42 to the track. When you add a booster, it can also be connected to another section of the PM42 and to the track powered by that booster. In this way, if there is a short in one section of the railroad, you can still operate trains or devices in the other areas.