This is so awesome that it makes me want to break my rule of not installing new plugins until they’ve been tested for a while! 
One quick suggestion regarding wording: I think it would sound much more inviting if the message title said “You’ve been invited to a mingle” instead of “You’ve been matched for a mingle”.
And some ideas for further development:
- It would be very useful to match pairs from different groups. (e.g. on one of my sites users are members of groups to reflect their background (from what perspective they are interested in the forums subject matter) and while it could be useful to match people from the same background (strengthen bonding social capital) it would also be great to match people from different backgrounds (strengthen bridging social capital).
- It may be useful to be able to specify the size of the mingle groups. I’m not sure about this because you may need to do it in diads in order to maximize likelihood that a conversation actually takes off (if you know, it’s only you and one other person, you may feel more inclined to respond than if you know there are others who can respond instead of you). But, given that the vast majority of any forum is passive, it may also be so that neither of the two matched users responds at all, so that nothing happens at all, wheras if you have, say, four people in a group, you have double the chance that someone makes the first step. And with three instead of one potential respondent, chances are that at least one of them responds. And once that has happened, chances are that the other two also chime in. It’s a matter of trying out, I guess, but my prediction would be that diads will only work if one or both users are rather active/committed users (which gives us yet another reason for suggestion 1 above).