I am not an attorney. But the Commodity Exchange Act has a fairly narrow window of fiduciary obligations for FCM's. The Agents of the FCM of course are liable for any representations they make to clients. But most of the CEA and CFTC legislation involves the segregation of customer funds when it comes to FCM's. The FCM customer documents talk about how big market moves might not allow the customer to get out of a position. TBH, in terms of "suitability" and risk most of the fiduciary onus is on the CTA and the Associated Persons in the investment fund. The FCM's responsibility for babysitting customer's risk in terms of "suitability" is really minimal.
If these clients signed documents directly with FC Stone, wouldn't FC Stone be in a direct client custodian relationship and would owe fiduciary duties directly to the clients? Then FC Stone would have a direct responsibility to ensure the suitability of the clients' investments in terms of riskiness and even to the IRA funds on margin in which many of those clients' transactions took place?