Tallin I think I view poly very similarly with the was you described it. I feel that my heart is big and I can "love more" .. like you, I havent acted on it but I feel as though I could.
Is it the right move for you, you ask? Well, if you've thought it all over (and you seem like you have, and it's a BIG plus that you've been open with your girl about it and that she is accepting and approving of it) and you believe it will enrich your life and allow you to share all that love you feel inside, and your heart is sincere... then, it sounds like you may have your answer already...
As for being open about your lifestyle if you choose to go for it, I think the only thing you can do is determine how much you will share with others depending on what you know of them and how you think they may react. It's risk, but, as the great Dr. Seuss once said... "Be who you are and say what you feel because those who mind don't matter and those who matter don't mind"
I believe that if we present our choices in a positive, sincere way, that people will be more likely to accept them. Of course there will always be those who find something to disapprove of. Ya can't please everyone! But, it's more important that you be true to yourself.
I hope everything works out for the best!