I've been working on this one myself for the past week and a half or so. My GF has been staying with us while her and her BF work some stuff out, and may be moving in with us on a semi-permanent basis if everyone decides it's the best move.
So far, it's been great. We're not out to our daughter yet, but she loves having her around. Lady Macbeth is excited about it, for the very least because she gets to spend more time with HER BF without feeling like she's abandoning me. I could easily see this being a permanent situation, with proper scheduling, &c.
Add in the concept that my GF and I have been talking about the concept of her having a baby (with me), and the playing house thing has been useful and enlightening.
We've only been together since October. So . . . 14 months doesn't seem super too-soon to me.