I can definitely still see poly happening. Not sure if it will, but I wouldn't count it out yet.
This point, in my opinion, would be a fairly awkward place to end the series so I assume/hope it's going to go on longer. And if it is then there should definitely be more interactions with and development for Rinna. Which would lend itself well to Mei getting further confused about the love that she's now feeling for Sayo by having her start feeling the same way with Rinna
Yea either that, or she starts realizing her love for Sayo first but then goes "hmmm some thing's still missing / not quite right though" and then that leads to further development with Rinna.
Given how Mei had no idea how feelings do, it would make sense for her to focus on one feeling first before getting to the next one