We would recommend against using different particle sizes and stationary phasess for guard and analytical column combination. That being said, the Cogent 2.o™ columns are valuable so it may be worth trying and seeing if the guard column does or does not have an adverse effect on efficiency in the method.
If the purpose of using a guard column is to protect against particulate matter only, then a precolumn filter may be more suitable. Remember that a guard column is a “chemical filter” and could potentially remove or adsorb compounds from your sample.
On a related note, a guard column with a different bonded phase should not be used with analytical columns except under certain circumstances such as lipid traps.