Yeah, I think it'd work on its own.
Ellie does,
Emmy does. Why not
Ally.
I think
Penny stands alone fine, too. I like
Penny. I'm just-okay with
Ally.
My thoughts on
Ally:
It reminds me of
Ally McBeal.
I tend to read it as "AL-lye" first - someone you have an alliance with? I think that's why
Allie might be a more common spelling.
I think
Alison is a great name, and since it hasn't fallen out of fashion rapidly, it's probably about like
Bridget in terms of being dated. I mean, not dated but becoming more classicky. Similar to
Jacqueline too.