Sylvie is so lovely. I also prefer this sweet French form to Sylvia. However, it sounds tongue-twistery with Elsa, unfortunately.
I feel like all the other names work well, and I love all of them too, Vivian being my least favorite, and I do find Alice too close to Elsa. Florence Wren reminds me of Florence Nightingale, though I do like the combo. Olive Helen is unexpected, but I'd find Olive Jane perfection.
I agree, Sylvie can definitely stand on its own! I personally prefer Sylvia, I just like that it's more classic and that I can still use Sylvie if I want. But it's not because Sylvie doesn't stand on its own, it's just that I like Sylvia better. :) I think Elsa and Sylvie would be charming together, I really love Sylvie Jane. I also adore Olive, but I'm not sure I love Elsa and Olive together for some reason. I do love the idea of Elsa, Sylvie, and Olive as sisters, though...