I think Ettie and Skye are lovely together. I do think Ettie and Olive go well but don't personally like Olive. I think Ettie and Penny sound a bit jarring together - I think the majority of ie endings won't work too well as Ettie has so many vowels. But some seem to fit.
Other ideas - Ettie and:
Alice, Aria, Amber, Ailsa, Arla, Alba, Astrid, Agnes, Autumn
Bella, Bethan, Bronte
Clara, Carys, Cerys, Cara, Ciara, Celest
Delphi, Dulcie, Dana, Dora
Elsa, Eva, Ella, Edith
Flora, Faith, Faye, Ferne, Fleur
Giselle, Grace, Greta
Hannah, Hope, Hazel, Honor
Iris, Ivy, Isla, Inez, Ida, Ingrid, Iona
Juno, Juniper, Judith, Jasmine
Kendra, Keira, Kayla, Kiki, Kristen
Lola, Lyla, Layla, Lena, Luna, Laura, Lois
Mala, Mila, Mika, Mara, Maia, Mia, Martha, Mary
Nuala, Niamh, Nora, Nia, Nyah, Nova, Nola
Orla, Opal
Pippa, Paige
Quinn
Rosa, Rose, Ruby, Rowan, Robyn, Romy, Rivkah, Reeva
Sara, Sasha, Saskia, Scarlett, Summer, Seren, Saffron, Sorrel, Sadie
Thea, Tia, Tanya, Tara, Tessa, Tamsin, Tamara
Una
Vada, Vita
Willow, Wren, Willa
Xanthe
Yasmin
Zara, Zia, Zeryn, Zelda, Zillah