Angelfielding, all companies you mentioned are reputable. I have worked for all of them. Just keep in mind what others said about the definitions of:
Clients: The ultimate customer, and our "employer," if you will. You are working for this client, e.g. XYZ gas station, wherever that shop is actually perfomed.
Shoppers: That's us...we do the field work...sometimes referred to as "field workers" by one or two companies.
Scheduling companies: Companies who locate shoppers and schedule shops for Mystery Shopping companies, who have contracted with clients to perform shops. Because of this, you will need to have an account with BOTH the Scheduling company (so you can view the shops they are trying to locate shoppers to perform) AND with the actual Mystery Shopping Company (MSC) who has contracted with the client to have the shop performed (so you can enter the actual report for the client at the MSC website). It can get confusing, remembering that XYZ Scheduling may have scheduled the shop, but you have to go to ABC Mystery Shopping to view/report on the shop.
The previous advice about a spread sheet is best. There is a generic form that exists. Contact Angela Megasko (aka JANE BOND) at "Mystery Shopping Solutions." She has a pretty complete mystery shopping spread sheet that I believe she might be willing to share. It's all set up with deadlines, fees, payments, expenses, etc.
Schedulers: People who work for Scheduling Companies OR for the MSC itself, scheduling shoppers to perform shops.
Editors: The people who actually READ and CORRECT your report before it goes to the client. These people also contact you directly for more information or clarifications, if your report has any such issues. They also GRADE your report, in many cases. ALWAYS RESPOND PROMPTLY TO EDITOR QUESTIONS...these are the people who hold the position of ultimately rejecting your report, if you do not satisfy their questions.