If your friends are poly, it is possible that your getting involved with one of the couples would not be considered "getting in between" anyone. Maybe they are open to others as well. Why not ask?
As far as "swapping versus poly," what you want could be accomplished within either a poly or swinging dynamic. Lots of swinger couples maintain good, close friendships and do more than just sexual activities with other couples they get it on with, whether in the same room or separately. You may want to check out some swinger sites or clubs, and go just as observers, and see if you meet anyone who is open to cultivating friendships with swing partners outside of the bedroom.
What generally makes it a poly Quad instead of swinging is allowing for the possibility of falling in love. In polyamory groups, you might meet some poly people who get into group sex but you might not. Lots of polyfolk do not participate in group sex. But lots do, there's no predicting. You may or may not have as much success finding a couple who would be open to your ideal - it depends on what sort of tone is set in the groups in your area. In many places, there are poly-kink crossover groups.
Last edited by nycindie; 06-12-2014 at 12:20 AM.