Wow. This is another one of those really interesting threads, for sure. I've no experience with this, but I'd absolutely discuss it with my doctor if I were considering it. She/he will know if thing in there will be in jeopardy from such activity. I can say from experience that sex with more than one person at the same time makes a better fantasy than reality. While it's one of those wild things you may need as a memory from your youth and all, I think multi-partner sex tends to end up being more about mechanics and keeping elbows out your face than the kind of dreamy emotionally deep sex two partners can have.
I'd also vote for keeping things tight rather than the fisting bit. Think about it. If you stretch things out to where you need a really large guy to be fully satisfied, you've eliminated at least 75% of the male populace as potential partners. I think the big penis/fisting thing is just a novelty popularized by porn because it's highly visual. A snug vagina and a small penis can have every bit as much fun as larger versions of the same. Both have the same number of nerve endings as their larger counterparts. The biggies just have nerve endings that are further apart.