Q: Shall I have special domain server for Ubuntu users? We've one ADDS server for 25 windows users.
Now we want to have 10 more Ubuntu machines for our developers.
So is right to add those 10 Ubuntu users to the same ADDS or we should build some other server specially for Ubuntu/Linux users ?

A: From what you've shared there's no reason to have a separate forest. Just add the 10 Ubuntu machines to your existing domain. At that scale the out of the box AD integration should be just fine. 
