Clothing and Covering the Aurah

Q: Is Salaah valid if a man reads in a tight fitting outfit?

Q:

1. What is the status of Salaah performed with short sleeves (shirts/kurtas)?

2. What is the reason for it being makrooh?

I heard that Nabi (Sallallahu Alayhi Wasallam's) under arm or to the effect became visible while in salaah due to the sleeves being broad or wide. The portion above the elbow became visible as with short sleeves.

Q: I have a thobe that is just above my ankles when walking normally. However when I bend down or go in to ruku, the thobe covers my ankles. Is it permissible to wear such a thobe?

Q: What is the ruling regarding performing salaah with short sleeves or a T-Shirt? 

Q: Should one fold his pants in order to expose his ankles at the time of Salaah?