In rocky planets,does fast rotation cause flatting or low flatting imply slow rotation? As far as I know, Venus and Mercury have 0 flatting, but Mars and Earth have detectable flatting, and Venus and Mercury are both rotating slowly. I'm confused as to the relation between rotational speed and flatting.

Does rotation cause a rocky planet to be flat? Or zero flatting implies Venus and Mercury rotate slowly? Which is correct?
The flattening of a planet is a function of both its spin rate and its structure. But for a series of planets of homologous structure the flattening depends on the spin rate. the faster it spins the greater the flattening (for spin rates typical of planets any way). So if a planet is spinning slowly it will display little polar flattening, similarly if it does not show flattening it should be spinning slowly.