Adobe Stock in Email Header

Well, yes, so long as you know enough about images to reduce the file size. Even a single stock photo is too large to email as it is sold.

Sure, the question is not if you can, but what licence you need.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).