Yes, once you obtain permanent residence, your status as a Legal Permanent Resident allows you to work for any employer.
Yes, any person can start a business in the U.S. without regard to immigration status, however, only persons with proper authorization to work in the U.S. can be employed by that business.
The transferring employee may bring a spouse and any unmarried dependent children under the age of 21, upon approval of their EB-1C visa application. A legal permanent resident may petition for certain other relatives for family-based immigration classifications.
Upon adjustment of status as permanent residents, spouses and dependents are permitted to work without any further immigration restrictions.