I use three online brokers and one is an FCM with whom CQG have a gateway agreement.
I do not quite understand your question re; charged as FCM. As far as I can recall, CQG charges a standard rate for use of the gateway which is the same for any of the FCMs with whom they have an agreement. All of the charges and conditions (comms, margin, lot numbers) from the broker remain the same. The CQG charges are for a "man in the middle" server which offers more server and Dom held order types (smart orders) and for some people perhaps faster execution speed. Of course their platform may be an attraction for those who like it.
I use broker supplied TWS, Onyx and J-Trader. Those three provide for all my needs without requirement to pay quite a bit of money for something I neither need nor would use.
Good trading to you as well 🙂