As others have said, probably ok, but you need to read the full set of instructions. It is easy to stop after "when not more than six breakers are installed" and miss the part that says "unless used as an appliance and branch-circuit panelboard".
The maximum six set disconnects per service requirement is found in 230.71(A) and I do not find anything about ?unless used as an appliance and branch-circuit panelboard". Could you state the section you are quoting, I would like to know my thinking has gone be the way side.
"230.71 Maximum Number of Disconnects.
(A) General. The service disconnecting means for each service permitted by 230.2, or for each set of service-entrance conductors permitted by 230.40, Exception No. 1, 3, 4, or 5, shall consist of not more than six switches or sets of circuit breakers, or a combination of not more than six switches and sets of circuit breakers, mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be not more than six sets of disconnects per service grouped in any one location."