One question I have, is it better to join an already existing family or start my own from scratch?
Only YOU can answer that.
What are YOU wanting/looking for? And what is your polysaturation point? Do you want to end up in a poly tangle? A polyfidelitous CLOSED situation? Of what size? Where is you polysaturation point? LDR or no?
There is no one way to do poly. I would suggest you spend time reading, then spend time looking inward to see what YOUR wants, needs, and limitations are.
I know for sure that when I was a single? I did not want exclusive and I did not want serious. DH was my FWB! And I was a hinge in a V, and still dating in the nondating dating way. I liked the fuzzy edges then.
Today? To open up from a marriage? Totally different wants and needs -- I want a CLOSED polyship -- of 3 to 4 people in it total and DH and I are already 2. I'm older, want things more clear cut, ship shape. I'm not keen on fuzzy as much. I do not want to mix raising families of children -- I prefer the children grown up and doing their own deal.
So spend time thinking about YOU and what YOU need.