Is there any specific steps to running in a development environment?
I assumed it would be as simple as dropping version 1.1.1 in the mods folder (run/mods), but connecting to any other versioned server results in this crash.

You are trying to run the release version within the development environment. Do you want to run the mod itself alongside the mod you are developing, or do you only want to depend on the API? I will add relevant instructions in the README.

How did you add it, did you add it with gradle or directly in your IDE?

I want to run the multiconnect alongside the mod I am developing

I just dropped the release jar into the mods folder, nothing extra. I am new to using fabric for mod development, so I am unsure if there are any extra steps needed.

I have added a section to the readme, please let me know if it is clear enough :)

Yep, following the instructions in the README allowed me to launch my mod alongside the multiconnect mod and I was successfully able to connect to a 1.13.1 server.

Thanks again!