I want to use manageiq as the cloud management system and WHMCS as the financial system to sell cloud resources. Could you please tell me whether these two systems can be connected?
Not out of the box.
ManageIQ is flexible enough to integrate with almost anything, there are a lot of points to hook your custom code into, but from ManageIQs point of view WHMCS is going to be any other Rest Service.
Anything more detailed would require me to know what WHMCS is and what your usecase is