Oddone's got it explained pretty well already =)
Mates are allowed, yes. So long as you keep everything between you two PG13, it's all good as anything explicit breaks our rules, even if it is "private" through group chat, parties, or whisper even. We want nothing to do with any of those things. They do not and will not ever have any place in this game as it is just not needed.
I handle Mate Centers by just watching them and the chaos that ensues lol I think I only legitimately joined a MC once back in Bonfire a couple months before the 1.16 update. It...it was an adventure lmao
I think it's alright if it's just fictional. Characters can have mates for RP and story purposes- there's nothing wrong with that. One of my characters had a mate before actually now that I think about it. I am uncomfortable with users using the term "mates" as they the players are together, though. That I see no need for personally.