Mature, well-rounded folks with a good grasp of themselves can handle just about anything they can dream up. People who already lack a solid foundation and necessary skills aren't going to have much luck juggling a dozen eggs or even just a few. Heck, some don't even get that far. It's sad, really. I also know folks who are painstakingly working their way up from a rather patchy past to develop skills they're missing.
People usually make their own arrangements through negotiation.
They talk about what they want and need, and why, and how they'd like to go about getting it. Then they settle on agreements that everyone feels content with. Open communication lets them deal with issues of jealousy, health and safety, time management, favorite and unfavorite activities, trust, and so forth. Polyamory includes a wide range of options from open relationships where both partners occasionally have one-night stands with other people, to closed marriages of three or six or however many individuals, to couples who sometimes like to swap partners with other couples, and much more. Whatever you can imagine, somewhere out there is probably a family that's figured out how to make it work for them.