1 HR per INN rule
 

In the men's league I work Monday nights, there is a limit of 1 over-the-fence HR per INN. Subsequent HRs are ruled a single. When there are runners on base, should they all advance one base if they are not forced?

I've been advancing all runners one base with no argument, since on a single, the runners would usually advance at least one base.

That's a local rule. Advancing runners should be addressed as part of that local rule.

Since the rule is not "all runners advance one base", I guess they must be forced.

If it's ruled a "single", then if not otherwise stated it implies that all runners are to be advanced one base. This is analogous to a ground-rule double wherein all runners are advanced two bases.