I like Veronica. It's interesting without being weird, well-known without being overly popular. I know a little girl about ten or twelve named Veronica. She is sometimes called Rocky by her family. I used to work with another Veronica, she'd be maybe 40, I don't think she had a nn.
I'm not crazy about Roni, but it can be kind of cute. I can't stand Vera, it puts me in mind of a blowsy, bleached-blonde, gum-chewing, chain-smoking waitress in a crummy diner.
If you want to use Ada (which I think is old and unattractive) I would just use it without ruining a pretty name like Estella. Estella is too nice not to use as a given name.
Lola is gross too.
Maya and Veronica make a pretty sister set.