Quote from trefoil:
I don't know about the SEC, but TOS allows you to do anything you want, except sell options naked, if I recall correctly. May have changed under TD, don't know, but I'm sure previous account holders were grandfathered on that.
However, the OP should note that shorting is quite a bit riskier and less profitable (in the aggregate; those who are good at it would of course dispute this) than going long. Lots of reasons, but to (over)simplify, the market has a long-term up bias, which means you're betting against the house if you go short.
Regardless of whether you're allowed or not, long-only is better if you're just starting out.