Probably the thing to keep in mind here is that the more specific it is what you're seeking, the longer it's likely to take to find the right person that fits the plan. You have to decide what trade-off is best for you as far as how long you're willing to search, and how specific you want the outcome to be. Many people just fall into a poly situation, not having a plan for structure at all ahead of time.
Re: coming out to your family ... I don't know of any special way of doing that to stave off negative reactions. It's really not up to you to convince people to react positively (or even reasonably), it's up to them. The best you can do is explain the situation to family members using respectful language, and being prepared within reason to answer any questions they may have. Sometimes people just have to freak out about it for awhile, and then very slowly they might get used to it.
Sorry that's the best I can offer in that area.
Otherwise, you should just continue to read and study and post any additional questions. You need to be flexible with whoever you meet, but I think you know that.