Hi Oscar,
+ Media transmission, ie sending/retrieving Photos and Videos (What Seaside modules I can use for this?)
You probably don't want to serve medias with Pharo but use Apache/nginx/whatever. They serve static files really well and you don't want to put load on your Smalltalk for such a task.
+ Database communication (I can use GlorpDBX)
Never used it but it seems to work fine for many people.
+ Ouath2 authentification (??, I understand that CouldforkSSO is a client not a server)
I would be interested if you find a solution/package.
+ Scalability, where I can easily grow from hundreds to thousands of clients.
If you go stateless (you probably will if you go full REST) you can distribute load on as many images as you want.
Do you think Pharo could meet that requirements or I must use something less cool :-(, such as ruby on rails.
Ruby/Rails has probably a bigger ecosystem and conventions around it but Pharo is definitely cooler ;)