Hi! I can't give you a definitive answer, but I'm part of IcyTea's discord server and have access to the alpha version of part 2. At the moment, you can't have kits with Quailtuft, and I doubt you'll be able to due to it not really working with the plot. However, (at the moment at least) the kits’ only inherit colours and markings from their parents.
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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. :)