Demasiado

Demasiado
 

Does it ever change to demasiada?

Example:

Ella es demsadia delagada

or does it stay demasiado?

Not in this case, (adverb)
Ella es demasiado delgada. (She is too thin.)

But, (adjective)
Hace demasiado calor.
Tenemos demasiadas horas de clase.
In the sense of (too much)
Más de lo debido: ‘Está demasiado caliente. Vas demasiado deprisa’. (Excesivamente. = in excess)
dar demasiados golpes,
ir demasiado lejos,
dar demasiadas vueltas.

In Latin America, «demasiado» is used instead of «muy» or «mucho»:
‘Es demasiado guapa. Son demasiado amables’.
She is too [very] good-looking. They are too [very] kind.

Question so much can be used in this case right?

It depends.

You are so much skinny is wrong, but you can say "You are so skinny, you have to eat more"

I love my dog so much is an example of so much.

All right.

I will practice more in the phrases, thank you for your help.