Here is a snippet from Nofi, showing 2100 men in 4 battalions and 5 squadrons with 3 guns.: https://books.google.ca/books?id=ZPFtsn-nRTwC&pg=PA320&lpg=PA320&dq=ghent+1815+french+berry&source=bl&ots=01Au2wKqI_&sig=BzjEeaiZRT0pUHTskRQwGZBc28Q&hl=en&sa=X&ved=0CBwQ6AEwADgKahUKEwith9-0y4fGAhWKSJIKHa-6AIg#v=onepage&q=ghent%201815%20french%20berry&f=false
Wellington did not view Berry as being under his command, but rather that the émigré army was the bodyguard for Louis and necessary as such.
There is evidence that Wellington prepared for a successful French thrust on Brussels by preparing depots supporting a line of retreat north from Brussels, not to Ostend. Would Louis have fled to Britain or followed the field army north, and what would have befallen his little émigré army? This is what Gregor Dallas writes: https://books.google.ca/books?id=XRxMWrm0ra0C&pg=PT125&lpg=PT125&dq=ghent+1815+french+berry+cavalry&source=bl&ots=wN2V9B8zWn&sig=7dAKRhhhHByz6wXUM_Eutw8ZSeg&hl=en&sa=X&ved=0CB0Q6AEwAGoVChMIgtbh8dGHxgIVTXuSCh0V-gBx#v=onepage&q=ghent%201815%20french%20berry%20cavalry&f=false.
If anything, this little army served a good purpose for Wellington. Without it, he would have needed to detach troops for the same role. One should not confuse it with a fighting force though. It was small, had no logistics and would have needed to be replaced by other units if it went into the line. It was effective at what it did, which was local protection of Louis without draining other resources.