Hi and Welcome
I think there is no doubt that polyamory at the stage in life when you still want to secure resources and have children is a different undertaking than coming to it later in life when all that stuff is behind you. Many factors will come into play, not least of which is where you live. Is it an area where there is a visible polyamorous community?
Still if you think you are polyamorous I'm sure you wouldn't want to make yourself miserable being mono just to have children. My advice is to take things one current issue at a time and if it's meant to be things will fall into place at the appropriate time. Coming to grips with polyamory has its challenges for everyone. You will not gain anything by trying to deal with future issues now.
Keep reading and participating in this forum and you will learn how things work. Building a successful poly relationship requires communication, negotiation and compromise. This work done well should prevent anyone from being a "doormat".
Supporting a partner through pain caused by another relationship is tough as there is often little you can do and that's hard to cope with for some people, me included.