I personally have not seen a female tail wag when placed with another female though I suppose it could happen, I just don't recall seeing it. I have seen females tail wag when placing a male in their cage but then again I don't intentionally place another female in the cage when I'm attempting to breed them. Less than 600g is pretty small for a 3 year old female, but it's certainly possible. As I've said before I'll breed any female that's more than 3 years old, I think the smallest female I've successfully bred was around 800g. If their body is mature enough and they're in good shape it's not a problem, they just produce smaller clutches of smaller than average sized eggs.
From what you're saying though, it wouldn't surprise me at all if your small female turns out to actually be male.