I thank you all for your replies and I'm very grateful to find very heartfelt words being shared.
I've thought this over after reading every word in all of these replies and I'd like to share what I've found. Normally I glaze over threads (sorry...) and pull out the key points but this is a topic that is very near and dear to me. As a child her age I was playing video games near constantly. Games back then weren't what they are now. The gamer side of me is all for it. The parental side of me is on the fence.
My relationship with her has always been very open and positive. Any time someone at school has talked about 'something dirty' she's always come to me first and foremost and asked me about what they said, why they said it, what it means, and is it a good or bad thing? In return, I made her the promise that if she is old enough to ask the questions she is old enough to receive adult answers. Now of course I don't go divulging every detail about everything, and she knows that (we talked about it). She's happy she can come to someone and ask ANYTHING, and she appreciates that, unlike a lot of other adults in her life, she isn't given 'little kid answers'.
Discipline has never been an issue. She does what I ask, perhaps not timely in all cases such as cleaning rooms, but she does it and I'm confident after talking to her that she understands my reasons or at least understands that she may not really know why I'm asking something, but that it's important and she needs to do it. So when I tell her that she can only play XIV with me when I'm on I trust that she will do just that. I'm also of the type that if I'm in a linkshell or FC, I myself don't have a tolerance for extreme foul language or topics even when it's just me, let alone my kid. So I feel like that situation will be handled well.
Where the blurred line comes in is reigning us in. I'll play all night long. I'll play for 18 hours if I'm off. I know it. I don't because I recognize my tenancy to do that, but I've still done it from time to time. How can I trust myself to reign her in? I feel like if her and I play together and she really takes to it, I can see us setting out to do big things all the time and not going to bed and making adventuring an excuse. She'll go right along with it.
Then there's the problem of exposing her addiction. If she becomes addicted and grows beyond me game-wise, that'll obviously be bad, too.
So at the end of the day I've asked myself this after carefully reading all of your replies:
Do I trust her? Yes.
Do I trust myself to set rules towards playing with only me? Yes.
Do I trust myself to set strict time guidelines? No.
Do I trust myself to not become addicted and fueling an addiction for her? No.
So then comes the question, "How do I come to trust myself with setting and adhering to time guidelines and addiction prevention?"
I don't honestly know. If I set time restrictions on her, I will have to follow it myself and I don't really want to do that. If I try to throttle her, I'll again have to do the same for me.
Do I want to share my love for FFXI/XIV that much? Yes. How can I have the best of both worlds? I don't know.