Sunnah & Nafl Fasts

Q: If one has qadha fasts to still keep, can one keep nafl fasts or do you have to keep the qadha fast first?

Q: Can one fast the 6 days of Shawwal as your fard days that you have to pay in together or must it be separate?

Q: What is the authenticity of sitta Shawwal (the six days fasting after Ramadhaan)?

Q: If I keep the six fasts of Shawaal with the niyyat of qadha, will I receive the special reward of these six fasts? Also, is it correct that for nafl fasts, niyyat has to be made before subah saadiq?

Q: Is there any benefits for fasting in the month of Rajab?

Q: Is there any truth to the claims that Nabi (sallallahu alaihi wasallam) fasted on Monday because he was born on a Monday. If it is true, is it permissible to use this to say that one can celebrate his birthday?

Q: I have read that it is makrooh tanzihi for one to fast on only the 10th of Muharram (without also fasting on the 9th or 11th). My questions are:

1. What is the ruling if one could only fast on the 10th due to some valid reason?

2. Should he rather not fast on the 10th?

3. What constitutes a valid reason?

4. What's the meaning and ruling of makrooh tanzihi?

Q:

1. If a person has fasts pending for a vow that he has made without him having specified the date, can he fast these fasts of zul hijjah or should he keep those fasts first?

2. If he has to fast for his vow in these 10 days will he acquire the sawaab of the zul hijjah fasts? Also if a woman fasts ramadaan qadhas in these days does she get the sawaab of zul hijjah fasts?

Q: Is it true that one cannot fast on 11th, 12th and 13th Zul Hijjah? 

Q: If a female has Ramadhan qaza fasts to make up, would she get the reward of specific nafl fasts (e.g. arafah day) if she chooses to make qaza of farz fast on that day?