Pascal's question about what licenses/certificates are required to start a taxi company and from which government department(s) he gets them still needs an answer.
However, I am in agreement with Paul Thate that it is highly unlikely that an outsider could establish a successful taxi business. There are already many companies providing this service with more than enough drivers to meet the demand. For an outsider to be successful, he would have to take business away from the existing companies--either ongoing business or some new service that could be provided by existing companies. The union(s) that controls taxi's is a very strong one and enjoys great political power, especially with the present government. The deck is stacked against the newcomer.
Anyone desiring to come to the Dominican Republic and start a business would be advised to look for something that didn't require going head to head with powerful Domininican interests. Fighting with other foreign businessmen for business is one thing, fighting with Dominicans, particularly those with strong political muscle, is quite another.