“it’s not a toomah!”

I’m on both and intend to find out. I suspect that is not possible though. You can follow your Funkwhale account from Mastodon, yet you can’t lookup and follow any mastodon accounts.

Create your own community (aka - matrix space/room), set up end-to-end encryption, start the invites. https://matrix.org/docs/chat_basics/private-group-chat/

Otherwise, to find existing rooms, I’ve relied on this in the past: https://matrixrooms.info/