Eko, it sounds like you two are definitely going about this the right way - trying to deal with the mental process of a mono and poly being together is a huge challenge. As you see from some posts here, some people don't understand what it is like to think of relationships as a monogamous person does - they don't understand the source of your fears.
This is the main issue, based on my experience - if poly folks haven't had the experience with a monopoly setup then they aren't going to be any more able to give advice than mono people who have never encountered poly. It's not a personal failing in either, just not a situation they have been faced with.
What seems simple to a pure poly or a pure mono is incredibly complicated when you get into a mono/poly situation. You DO need to talk things through to a FAR greater degree than with a mono/mono or poly/poly situation. You DO need to spend more time trying to work things through, because it is so much harder to put yourself in their position. The danger, of course, is that you overthink things, but to be honest it's better than just "going with the flow" and experiencing a major train-wreck where everybody gets hurt.
"Listen, or your tongue will make you deaf." - Native American Proverb