With Ashby, which has a somewhat unisex, surnamey type vibe to it, I think Harper would be the best match stylistically. However, I'm not sure about Harper Holloway, it just seems a bit cartooney to me.
Your other names are fine with your surname, but they just don't seem to pair up too well with Ashby, without making her seem like a boy, if that makes sense - they're much more feminine, and I'd expect Ashby and Matilda to be brother and sister rather than two sisters, while Ashby and Harper are more similar to be sisters, if that makes sense.
Other suggestions, using your names as middles:
I think Ashby and Mabel have a sweet ring to it... maybe it's the "B" in both of them?
Ashby Kate and Mabel Jane.
I love Mabel and Mercy -- actually all of these are great choices. Mabel Eliza would be cute.