The phrase "as a rule only" is correct and usable in written English.
It can be used to indicate that something is generally true or applicable, but with the limitation that it may not apply in all cases.
Example: "As a rule only, we allow one guest per member at our events."
Alternatives: "generally speaking" or "typically only."
As a rule, only the female cares for the brood, and males live only long enough to mate.
Fewer and fewer roles are being advertised in the printed press — as a rule only the most senior positions make it into print.
In 2006, when Ms. Wall Spitzer showed a reporter around their apartment on Fifth Avenue, she proudly declared that as a rule, only artwork made by members of the immediate family was allowed to hang on the walls.
In the class Malacostraca, which includes the decapods, there is a consistent body plan: the trunk (which follows the head) is divided into two distinct regions, an anterior thorax of eight somites and a posterior abdomen of seven somites, although as a rule only six are evident in the adult.
As a rule, only about 5% of the food at the surface is available to creatures lower down, and four-fifths of that is captured before it reaches the sea bed.Because many deep-sea creatures make their own light, there is still a need for disguise in the dark deep.
In the surface layer, as a rule, only semi-volatile products of transformation were detected which was caused by fast evaporation and biodegradation of volatile metabolites.
Of course, directorial "authorship" doesn't happen as a rule but only when a director exerts sufficient control over a movie to make it so.
There is no single way to define which is which since each set of circumstances can vary drastically, but as a general rule, only break a rule that needs to be broken.
As a general rule, only the main statement of a case such as this is publicly available, but not a list of accompanying documents, or the documents themselves.
As a general rule, only turn to family and friends for loans as a last resort.
("As a general rule, only the very smartest people can make truly catastrophic mistakes").
