can airmail stamps be used regular postage?

Answer 1

Yes, assuming you are in the United States. Airmail stamps can be used for any class of mail, as long as the proper rate is paid. And it doesn't matter if you overpay - just make sure there is at least 39c on a one-ounce letter within the US, 63c for a letter to Canada or Mexico, ot 84c to the rest of the world. For letter mail, there is no separate airmail rate - all mail goes by the fastest method. There is a separate airmail rate for international packages, which may go surface otherwise.

Answer 2

Yes
Just be sure you have enough postage and write "air mail" on the front