... a lot of our policies have been set up for years (from 4.5, thru' 5.1 to 6.5.1) with a mixture of frequency-based & calendar-based schedules as follows & run successfully:
Mon-Fri Differential Incrementals (frequency based)
Sat Cumulative Differentials (frequency based)
Monthly Full (Last Saturday of month & calendar based)
so have always wondered about this not mixing issue!
***Edit***
Coincidentally, you may find the following blog & link just posted by TimBur interesting:
Random Links