First, let me correct myself (I'm very sorry for that) but I was wrong regarding the organisation of the Prussian infantry. From 1787 until 1799 the infantry regiment had one grenadier battalion of 4 companies, 2 musketeer battalions (each of 4 companies) and one depot battalion (of 3 companies?). So combining musketeers and grenadiers into one battalion wasn't used.
Only from 1799 onwards, the infantry regiment had just 2 companies of grenadiers which were combined into one grenadier battalion with 2 grenadier companies from another regiment. The regiment also had two battalions of musketeers but now of 5 companies although in the field they probably were organised in 4 "field" companies.
As said above, organisation changed in 1799 but that was caused by the new king William III who in 1797 came to the throne. Not only organisation changed but also the uniform. The 1787 kasket (casquet) of musketeers and grenadiers changed into a form of tricorn (for musketeers) and the higher grenadier hat (for grenadiers). Also the uniform changed into the closed to the waist kind; in fact the 1806 uniform of Jena/Auerstadt period.
I have tried to find other armies who used the Prussian grenadierhat but haven't found noe up to now. It is a peculiar model so i doubt it will be used by any other army.
Greetings
Jan