Using the Two Pedals at Once
May the damper pedal and the soft pedal be used simultaneously, or would this be detrimental to the piano?
Since the mechanisms of the two pedals are entirely separate and independent of each other you may use them simultaneously, provided that the character of a particular place in your piece justifies it.
To Produce a Softer Tone
Should the expression "p" be executed by the aid of the soft pedal or through the fingers?
The soft pedal serves to change the quality of tone, not the quantity. It should therefore never be used to hide a faulty piano (or soft) touch. Mere softness of tone should always be produced by a decrease of finger-force and a lessening of the raising of the fingers. The soft pedal should be employed only when the softness of tone is coupled with a change of colouring, such as lies within its range of action.
Do Not Over-Use the Soft Pedal
Should the Gavotte in A, of Gluck-Brahms, be played without the soft pedal? Does a liberal use of the soft pedal tend to make the student lazy in using a light touch?
Your first question is too general, as there is no piece of music that should be played entirely with or without the soft pedal; it is used only when a certain change of colouring is proposed. A too frequent use of the soft pedal does tend to a neglect of the pianissimo touch, and it should, therefore, be discouraged.
Once More the "Soft" Pedal