This is pretty factual. Though the rumour wasn't that he had sold his soul to the devil, but that he was the son of the devil. So much so that while he was working in Prague, he had to print letters from his mother in the The Quarterly Musical Magazine to prove that had human parentage.

He's also described as having black hair, being very thin, very tall, and very lanky, and there's been speculation as to if he had Marfan syndrome, a condition that gives you greater height, as well as elongated limbs and fingers, although others have speculated that he had other connective tissue disorders which led him to have extraordinary flexibility in his fingers.

As for the breaking strings, he wrote a piece called Variations on One String, which is supposed to be played on a single string, and in a way of showing that off, he would break the other strings prior to playing it. It apparently started off as somewhat of a bet from Paganini's perspective:

"Looking for variety in the programs I executed at court, one evening — after having removed two strings from my violin (the 2d and 3d), I improvised a sonata entitled 'Scena amorosa,' the 4th string representing the man (Adonis) and the treble string the woman (Venus). This was the beginning of my habit of playing on one string, as this sonata was much admired, and I was asked if I could play on a single string. I replied : 'Certainly,' and forthwith wrote a sonata with variations, which was performed in a grand concert on Saint Napoleon's day. Subsequently I wrote several sonatas in the same style."

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.1780251119 https://archive.org/stream/nicolopaganinih00stragoog/nicolopaganinih00stragoog_djvu.txt http://www.worldcat.org/title/paganini-the-genoese/oclc/890638 https://archive.org/stream/nicolopaganinibi00prod/nicolopaganinibi00prod_djvu.txt