Please Help ASAP----HR in Construction Firm

Hi...

I just accepted an HR post in a construction company...since i am new to the industry i have so many questions regarding benefit administration:

1) Is SSS, HDMF & Philhealth mandatory even if the engagement is project based only, minimum of 1 week; if yes, what will be the basis of computation?
2) Is it mandatory to pay them 13th month?if so how?we have project based empoyees contracted only for a specific period of time and engagement is not continuos...
3)Our OT pay is just the per hour rate and we do not give premium for it, however the project based employees are aware of this computation and they accepted it as stipulated in their signed contract...

I hope someone can help as I feel that being an HR i have the responsibility to correct whatever wrong practices we have in my company...

Thanks...

even proj employees entitled to sss, hdmf, philhealth,

13th month based on what their annual earnings divided by 12 regardless of length of project

even if they accepted the non-premium, it still isn't lawful