Well, from the post, I would say that they are off for a good start. She put together a list of motivations, and she said she was already "naturally predisposed" for that sort of thing (I would guess a jealous person, or someone with strongly rooted convinctions about monogamy woulnd't have ever thought to give it a try, and would have just said to MBlume "no thanks", walked off, and tried to find another suitable partner). She might not have thought of poly in the first place, and the original motivation to enter into this kind of relationship might have had more to do with her desire to date MBlume (to her, it must have been a rather serious perk: she decided to go live in another place, she changed a rather important part of her life) than with her innate curiosity about that kind of life-style, but judging from her initial outlook, even before deciding to give it a try she didn't seem too adverse to it (I would say she might even have considered it anyway, some years down the road, given the right stimulus). And even in the events of things ending badly... well, it's not as if she couldn't go back to the way things were before.

I will say this: she didn't mention any jealousy, on either part, and the "ground rules" she put in place seem to have reassured her of her status, so I would say that her odds are pretty good. The fact that she feels confortable enough with her boyfriend to tell him "stay home with me, tonight" or to put down some rules about marriage and the prospect of children seems to indicate that they have pretty good communication, which is the most important thing anyway (the situation might have been different had MBlume's girlfriend been in a primary relationship with him, at the time, because then Alicorn could have ended up in a "subordinate" position and I guess she woulnd't have enjoyed being the third wheel).