I really don't like it. There are feminine forms of masculine names that seem totally natural to me, like
Josephine or
Adriana, and then there are some that seem incredibly forced, like Thomasine or
Jamesina. My first instinct would be to pronounce Thomasine as to-ma-SEEN and
Thomasina as to-ma-SEE-na.
Despite my dislike of Thomasine, I can see it making a really nice middle name for
Kaia or
Kira. I think I like the
Kaia combo a bit better.
Well, if it's only her middle name, I doubt she'd get called
Tommy. If it was her first name, probably. But hey, maybe she'll like it.