Live and learn. I wouldn't be too worried about Service Sleuth, they do pay without being threatened or cajoled. It is just that their payment cycle seems unreasonably long. As for Restaurant Evaluators, I don't know anyone who is still working for them, so please keep us posted how that works out.
There is no "right way" to start. I generally suggest that folks start with Market Force (blue portal), Bare, Corporate Research International and Maritz primarily because they have shops everywhere, are accustomed to working with new shoppers and generally provide clean instructions. But a major factor of that is the availability of work. Judging from the E mails I'd get, there are lots of jobs in the Miami area and evidently not enough shoppers to do them. So frankly you could sign up with virtually any company discussed on the board and probably find work in your area. Good luck and welcome to the forum!