So I want to use Adafruit’s servo bonnet, a few other bonnets and a couple of sensors with my raspberry pi zero.
But what are you going to use them for ?
is there a way to connect multiple raspberry pi’s together to run as one and use all of the GPIO pins?
Not really, you can make a distributed application, different programs on each Pi using MQTT for example to exchange information between themselves.
I've also seen mentions of "pigpio" which is a networked service that also give you access to GPIO pins over the network. If that is something that will work with the sensors and bonnets you plan to use I cant know. ( http://abyz.me.uk/rpi/pigpio/
Clustering is mostly
about dividing computational tasks between different CPUs.