Currently, you can't progress Quailtuft romance past the 'dating' stage that is already available in this version, and I highly doubt that will change. So you can't become his mate or have/adopt kits.
And yes, you can become mates with characters outside of CedarClan! There is Cinder (who you can only romance if you join Rouge's Domain) and Nico. I believe you can also date one of the prisoners of the domain, but I'm not sure if you're actually going to be able to become mates with him. :)