Free photos of Adobe stock

Assets from the free collection come with a licence. That licence determines what you can do with the asset. If you licence a free photo, you'll get one standard licence, that you can use according to the standard licence for commercial and non-commercial projects. 

However free assets do not have a "licence again" or a higher type of licence (extended!). So you can use a free asset for one customer only and you cannot use it for mugs, t-shirts and other stuff. 

I wouldn't build a carear on the free assets, however, as there is a limited choice, and if a customer is paying, they deserve the best solution to their request. 

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).