I don’t think the minimal iso include cloud-init. Actually, even the AWS and OpenStack images don’t use cloud-init (closure size mainly): during the boot, they use curl to get back the cloud-init metadata and configure hostname and ssh keys. So, these images support a small subset of cloud-init (but a lot a people consider other cloud-init feature doesn’t fit with the declarative nixos configuration).
In your image, you could however easily activate the cloud-init service.