There are some fairly tried-and-true rules in the Swinger Lifestyle (regarding: respect, communication, honesty, trust etc.), but I suspect the question of: "Should we try and include swinging with our vanilla friends?" is largely situational.
Based on my general memory of what I've seen posted on this BBS over the years, I'd be inclined to say that "it may be a bad idea". On the other hand, if it worked, it could be great!
I think the real question that everyone in this situation has to ask:
"Is it worth the risk (to the friendship) to give it a try?"
:-|
I looked back on your previous posts in this thread, and it appears to me that y'all are approaching this with your eyes wide open, which is a good thing. I suppose there is no way to know for sure (as usual) till you actually try it (and jump off that proverbial cliff).
Emphasize the basics, and your chances of success should increase exponentially.
As was mentioned previously, sticking with your rule of "no friends, no co-workers" will always be safest.
I would think that one other significant factor would be: "How strong a grasp do all involved have regarding what really makes a swinging situation work?"