It is only two syllables so I don't think people with shorten it. I wouldn't worry about it.
I like Flora. It's very naturey and pretty and I think it's spunky enough without a nickname. I don't think Flo is going to be a problem because not only if Flora so short anyway, Flo and the Flo- of Flora are different sounds.
Florie (like a pp said)
Flor (like Fleur)
...What about Flossie, how do we fell about that?