As to having dreams - no, it's not crazy. Something being highly unlikely doesn't equal it being crazy. I used to dream about becoming an astronaut, and fantasize about how COOL space-flight would be, but never took any action beyond day-dreaming to further my career plans.
A goal is different from a dream in that you actually set specific times in the future you will measure against your progress, as in "In three more years, I will graduate" or "After I land a steady job, I will buy a house". You take active steps to work towards your goal.
It is perfectly okay to have as your goal the dream to live in a polyamorous household. It's hard to achieve, but so are most things worth achieving. The best preparation I can think of is to read those people's experiences who actually live with multiple partners ("Multi-partner cohabitation" thread is a good place to start, as are member blogs), and learn from them what to expect, what has worked for them and what hasn't and why.
Other than that, when going out on to the dating field, it's good to be upfront with prospective partners and tell them as soon as possible that you don't want exclusivity. It's not necessary to spill your whole life's dream onto them on a first date, just make clear you are not looking for traditional monogamous relationships.
As to having thought down specific numbers, genders, children, races and places to live, well, that is not something you can control. Many people struggle with finding just one partner who is okay with poly, much less a second one. Finding four to five other people who are all interested in co-habiting abroad and having children as a group, with everyone equally interested in one another, is beyond unlikely. It's good dream material but not the stuff of real life.
Me: bi female in my twenties