I have F1 visa until 2020, i applied for TPS and have been granted it, but i haven't applied for the work permit yet. Is it advisable to apply for work permit under student visa? Has anyone done it? I am working 20 hours on campus but want to work more during weekends and free time.

Go and apply for it. Why do not you apply in the beginning while you apply TPS.

nsb - dherai jana le student visa ma TPS ko EAD apply garyo bhane OPT ma kaam garna paudaina ra tax pani dherai tirnu parcha bhanthe, so i didn't apply then, but now i feel i should work more hours to sustain myself.
TPS ko certification kehi aaucha? biometrics bhaako 3 weeks jati bhayo, kehi aako chaina

ani TPS ko EAD aayo bhane, OPT ko EAD valid huncha ki hudaina?

I think you can apply for TPS EAD anytime. Instead of that these days colleges and University are making exceptions to students from Nepal who were here during the earthquake time allowing them to enroll for 6 credits for Undergraduate and 3 credits for Masters. I heard that students can also apply for financial hardship EAD. Instead of TPS EAD which i heard if you work under than your status will be void and all that, why not financial hardship bhanera EAD and work full time. If you have 20 hours of work in school talk with your DSO to allow work more hours within school as well.