i would not talk to your ex first, and here's why:
if you talk to your ex and she is on board, then you have already created some expectations (or at least some anticipation) for both of you. you have already started rekindling *something* with her. then when you go to your girlfriend, she ALREADY feels like she is on the outside of it. if you guys were already in an established poly relationship and had had previous experiences, that might be different, but your girlfriends FIRST experience with poly would be "hey, i wanna do this and we're all ready to go and how do you feel about it?"
could you see how she might feel a little blindsided by that? that would put HER in the "weird" position of essentially having to say yes, even if she's not really comfortable with it, or risk either losing you or causing some serious resentment. THAT could "mess things up with the GF" way more way more than a no-pressure, conceptual discussion of polyamory.
it might instead be a better idea to bring up the concept of poly with your girlfriend first. give it a few days/weeks to sink in. make sure you talk about all the various aspects, pros, cons, etc. discuss all her concerns. hear her out. make sure that she feels solid about the concept BEFORE you introduce the ex factor in. yes, this will require that you wait a while before bringing the ex in, but it will set the tone for honesty and openness between you and your girlfriend.
and to be clear, there's nothing saying you can't discuss polyamory with your ex. send her some links, get her take on the philosophy and see what she thinks. just don't promise or even allude to something (a relationship or something akin to it) until you make sure your current partner is on board.
as for the positive gains for your girlfriend, that's a great question, and kudos to you for giving it thought. is she really independent? would she appreciate extra alone time to explore hobbies or personal interests? or would she resent having less of your time? could you all hang out together and do activities as a group? how does your current girlfriend feel about the ex? do they know each other? do they know about each other? not deal-breakers, but definitely something to think about.
another thing to think about that could be a potential "plus" for the ladies: would you be open to them dating other people as well? if not...at that point, what are they getting out of it? unless they're both bi and hot for each other, good friends (or potentially good friends) or both super super independent...well...i dunno, maybe just think on it for a while. put yourself in their shoes. that goes a long way.
sounds like you're on the right track...just don't rush into anything. best of luck to you and your ladies!