For the first example, the plural undefined article "unos/unas" can be omitted when the pronoun is present in the sentence when is used to assign properties to the subject. yet the meaning can be interpreted in different ways as using the article actually gives emphasis in the assignment.

on the second example, you are misusing the article "el" 

their use is the same as the "the" in English and using it the way you are doing it is transforming the meaning of what you are saying. 

the proper way for your example is to use the undefined Article "a/an", in both English and Spanish they have the same meaning and usage. 

> saying: "él es el maestro excelente" literally translates to : "he is the excellent teacher"  

implying that there's only 1 excellent teacher, to avoid this you use the undefined article for the singular male "un" 

>"él es un maestro excelente "