I think the answer to most of those is "it depends".
I'd highly recommend the documentary "When Two Won't Do" -- it takes an unvarnished look at a variety of polyamorous relationships, including that of the filmmakers.Stauderhorse answered the questions...
Not too shabby. Good to see word is getting out. A polyamorous person is someone who is more suited to having relationships with more than one person than she or he would be to monogamy, or someone who is practicing polyamory. Polyamory means more than two people are involved. That can mean all sorts of different relationships. The most simple is a "V" in which the persons at the ends of the V do no really have a relationship with each other, but they both have relationships with person at the "bottom" of the V, with the informed consent of all involved. Polyamorous relationships can be open or closed, so not every polyamorous relationship is an "open relationship." Many polyamorous people are in closed polycules and many aren't swingers, either.I read "The Ethical Slut" a few months ago and am a bit fascinated by the idea as well. I want to bring it up to my boyfriend, but I don't know how he'll take it and don't want to upset him, plus I'm not quite secure enough in my self-esteem to guarantee I won't turn into a jealous harpy. I knew a couple in college who were polyamorous, so I'll try to answer your questions based on my limited knowledge.
1. Are the relationships in a poly between a single person and lots of other partners, or are the partners in a relationship with one another too? (i.e. John and Jane are in a relationship then Jennifer comes into it, is she Jane's girlfriend too or just John's?)
A poly relationship is generally a regular couple with more people joining in, but if a single person just has a lot of partners, it wouldn't be considered poly. If those partners are in a relationship, it is poly. It's a bit confusing, but if there are at least two people in a couple with other partners coming into play, it's poly. If Jennifer dates Jane, then she's Jane's girlfriend. If John and Jennifer don't interact in a way that goes beyond friendship, John and Jennifer aren't dating, but Jennifer and Jane are.
2. Following on from that, do poly relationships cross sexualities? Is it common for a man to be dating a man an a woman simultaneously polyamourously? Or two women adding a man? Or more people in any kind of mixed-gender set up?
Anybody can date anyone. If both participants in a poly relationship are bisexual (as were the couple I knew), the man can date a man or a woman, two women can add a man, two men can date a woman. The only limit is what people are comfortable with.
3. Do polys typically form as relationships, or are some partners only there for sex? I can imagine the concept of a couple essentially having open relationships with boyfriends/girlfriends on the side, but is that more or less common than the other person being a member of an enlarged relationship?
Again, this totally depends on the people involved. If I was in an open relationship, it would basically be my boyfriend and I involved romantically, with some different sex partners thrown into the mix. Then again, people can date more than one person at a time if they're able to handle the emotions of everyone involved. I'm not sure which is more common, but I would imagine sexual relationships. One of the biggest fears of someone who is new to open relationships is that their partner will fall in love with a sex partner and leave them.
4. How do poly relationships begin? I suppose I could see it starting from an open relationship base and taking it to the next level with the third party being brought in as an equal partner.
Put two open-minded people together in a couple and odds are good that they will at least think about an open relationship. They might discuss it and lay down ground rules, then go searching for partners. Sometimes people who were mainly brought in for sex become so well liked by both people in the couple that the person becomes a romantic partner to both parties.
5. Is polyamory polygamy/polyandry by another name? Is it common for there to be lots of people in a relationship, or is it more likely just to be 3? Is there really a distinction between a man in a relationship with two stable female partners calling himself polyamorous and a man married to two wives who are polygamous?
Polygamy and polyandry refer to marriages where there are more than two people. Polyamory is more than two people dating or being involved. I would imagine that with more people being involved, there are more complications (time constraints, hurt feelings, details to work out), so there are probably fewer people in general. (Though the girl in the couple I knew had 5 girlfriends in addition to her boyfriend; how she found the time for all of them, I don't know). The only distinction between the terms polyamory and polygamy is marriage, I believe.
6. Is there a practical limit on a poly relationship? How many people can be in one? If you had a mix of the right minded people I could see lots of couples mixing together into one large poly group, or threes or fours or whatever.
There are no real limits on poly relationships. Hell, there could be a hundred people who sleep with and date each other in various configurations. But at what point does it cross over into just a group of friends who sleep together? There could be one main couple, but most people have a hard enough time finding one person to date, let alone dozens. It really just depends on what people are cool with.
7. Or am I overthinking this an it's all just poly - as soon as you have more than two people it's a flavour of poly in the same way that it's all icecream but just different flavours and toppings?
My defintion of poly is two people in a relationship who also date others; i.e. a couple who isn't exclusive. There are many shades of poly that all fall under the same term; I recommend reading "The Ethical Slut" as it gives some basics as well as potential pitfalls. An increase in people usually equals an increase in drama, but if poly people are willing to deal with all that, more power to them.
Polyamorous people face discrimination and bullying. They should not be denied their rights. The reality of polyamory is one reason we need full marriage equality and relationship rights for all consenting adults.