I'm trying to devise a solution that will connect multiple Raspberry Pi zeros together for communication between them. I'm using a touchscreen on each of them, so GPIO pins 1-24 are taken. Also, I'm using the RPi zero model without wifi. Would I be able to use pins 25-40 to communicate? Can I use SPI on these pins? I would prefer a daisy-chained solution but hub and spoke is ok if necessary.
You still have USB on RPi Zero, meaning you could attach any USB communication dongle you want (including WiFi or Ethernet) and use the appropriate networking architecture. Daisy-chaining is not out of the question with Ethernet, but that would require 2 network adapters per RPi, so a hub+nodes architecture will be more efficient.