Al-jawab billahi at-taufeeq (the answer with Allah’s guidance)
Wearing the shalwar kamees, if it fulfills the requirements of covering the `Awrah, then it is acceptable; otherwise, it is not. Any substance that conceals the colour of the `awrah under normal light is acceptable.
So therefore wearing the shalwar kamees outdoors is totally prohibited. Wearing it underneath the Hijab/Niqab is permissible.
As for propriety and dignity, at home women should dress in a modest manner, preferring loose unrevealing clothing.
It must be remembered that to wear the Hijab/ Niqab outdoors is compulsory and indoors if there are Ghair Mahram men (those whom you can marry) present.
Go to these sites to understand more about Hijab/Niqab.
And Only Allah Ta’ala knows Best.
Moulana Qamruz Zaman