Can I use a 12MHz for NTSC video generation
To answer your actual question, no, "modern" TV sets will not be very forgiving.

Older TVs and monitors (analog signal path, CRT display) were very forgiving. Anything that had approximately the correct vertical and horizontal sync frequencies would display something useful.

Any modern set (i.e., digital signal path, flatscreen display) will be sampling and digitizing your NTSC signal with a 13.500 MHz sample clock so that it can be scaled and displayed. If your pixel rate isn't close enough so that the TV can synchronize to it, the pixels will be "smeared" across multiple samples and will look fuzzy.