You will most likely need an Independent Contractor Agreement between you and your contractors and a Services Agreement between your company and your customers. Here are some of the points that an Independent Contractor Agreement should include: (1) the services your contractors will provide, (2) how and when you are going to pay them, (3) what types of insurance they are required to have, (4) how your liability is limited, (5) that they contractor is a contractor and not an employee (and why), (6) termination and (7) confidentiality/non-circumvention/non-solicitation of your clients. These are the big-picture points that should be included, but a lawyer will draft an agreement to make sure that you are protected as much as possible under these agreements and also that the deal between you and your clients/contracts is clearly defined.
Ms. Olmon has provided excellent information and Mr. Spiritos' advice is also excellent. You should consult a business attorney on these issues.
Since you are using a separate and fully licensed business to perform the work, you should not have any difficulty with the independent contractor status and payments, as opposed to hiring workers as independent contractors to perform the work, but you will also be viewed by the customer as performing all of the work even though you are using an independent contractor, unless you disclose that you are brokering the work out.