Can I legally hire an employee using a itin number in San Francisco?

Not if this person is not otherwise authorized to work by the USCIS.

After more than a year wait, I got RFE(regular) on December i140 2015,(ability to pay) and we send company's tax files,

Premium processing availability for an I-140 is limited to only certain circumstances. Consult with an attorney.

Can a green-card holder vote in Democrat Primaries in California?

No. Most certainly not and to do so could cause you great problems.

Should I provide address of were I stayed while traveling to the USA on a trip?

You should provide the most complete information as possible.

Can I send a job letter with current pay slips to prove I meet the 125% poverty line

Yes, that can be used as proof of current income, however, you must strictly comply with the I-864 requirements.

How can my brother retain his student status in a cheaper way and still remain legal?

F-1 status is restricted for purposes of public high school attendance. Consult with an attorney.

Sponsor for Adjustment of status

If you can find a willing US Citizen or Lawful Permanent Resident to be a co-sponsor, this should not be an issue.

Yes, there will be issues and you must be able to explain/document the merger at the interview.

I was wondering if I can travel to Houston,Texas since it filed a lawsuit to President Obama for its executive order.

Texas is against DACA, but that does not mean you would be in danger if you are in possession of adequate documentation of your status. Consult...

Am I automatically a USA citizen of my mother was born in the USA and my adoptive father was born in USA too

You could file an N-600 application for certificate of citizenship, assuming you meet the requirements. Consult with an attorney.