Is a fiscal conservative and social liberal a RINO?
Depends on how socially liberal. Scott Brown seems to take soft positions on abortion and gay marriage, but to deem him a RINO because of them seems just a tad overblown. As I noted above, Scott Brown's stated position on abortion, if pursued to its logical end, is more "pro-life" than that of many of these hard-core pro-life activists - for the simple fact that it would save more little babies' lives than they would.
As for gay marriage - well, his position is the 10th amendment position. If you have a problem with that, then you ultimately have a problem with the Constitution, not with Scott Brown.