Niyah for Nafl Fast along with Qadha

I have learnt that one cannot make the niyah of a nafl along with a qaza sawm but for the sawm whose days are specified (e.g. Monday, Thursday, Yawm-e-arafah, Muharram, 15 Shabaan but not that of shawwal any that's any 6 days in the month of Shawwal), one can do qaza and one would automatically get the reward of that specified day if one has intended it as the aim is to spend that day in sawm, so just like how if one spend the nights of Qadr in doing Qaza salaat, one would get the reward of spending the night in ibaadah as the aim is to fill these auspicious nights with ibaadat. Please explain with reference.