I have several core programs installed on my SSD but I store their output files on a traditional harddrive (i.e. Photoshop is on my SSD, but I store the output on my Seagate 7200.12, MS Office is installed on my SSD, but I save my Access / Excel files to my Seagate drive, etc).
You pay a lot of extra money for an SSD, and what you're paying for is performance.
Torrents and other Internet uploads/downloads are limited by the speed of your Internet link, which is a LOT slower than an SSD. It won't hurt the SSD, but there's really no reason to put it there - those types of files will work just as well on an HDD. Might as well put them there and save the SSD for performance-critical stuff like the OS files.
I heard that SSD is very bad for p2p apps, such as bit torrent. Is it wise to have a SSD as OS drive on a PC that bit torrent is used a lot on it?
Will I be fine if the target of download is set to the traditional HDD?
I have an SSD and it makes no difference for downloading, as your write speed is still going to be much much faster (even to an older HD) than the download speed going to the disk. The biggest difference (outside of the enormous cost of a sizeable SSD) is that the read speeds are much faster than a typical HDD. Apps on my system nearly all open instantly and load times for games are almost non-existent.