Probably.

The prohibition on the Istari:

From "The Istari", Unfinished Tales - "their emissaries [the Istari/Wizards] were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men or Elves by open displays of power"

LOTR, Appendix B - "But they were forbidden to match his [Sauron's] power with power, or to seek to dominate Elves or Men by force or fear".

Fighting Smaug wouldn't constitute trying to rule or overawe Elves or Men, nor trying to defeat Sauron directly, so it would probably be OK.

Probably.

The prohibition on the Istari:

"Their emissaries [the Istari/Wizards] were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men or Elves by open displays of power"
_{Unfinished Tales, The Istari}