To my knowledge for a community to show in [All] at all someone from your server has to had looked it up. So I used the feddit communities catalog to opt us into all the cat communities. No need to thank me. Sorry if it kills our instance. ;_;
There is too many. I'm still seeking any I might had missed. They should appear for you if you go into communities and choose all and then type stuff like cat/cats/paws/aww/illegally smol etc. It will be all easy for you to find because it is now indexed for us. I have to continue my mission to add them all. Sorry!