I think preparation for this specimen first comes down to; what do you want the outcome to be? Are you hoping to remove all of the matrix around each individual specimen? If this was for my personal collection, I would probably keep the entire block intact and just scrub it down with a toothbrush and some water. Maybe chip away some of the limestone around each individual specimen to reveal more of the face using a carbide tool or air scribe (I’m not sure how hard this matrix is). If you have no time constraints, I know people who just place these blocks in their yards and allow the rain to do the work. I have never used acetic acid in fossil prep, but when we use formic acid, it is a 7% concentration mixed with water. I’m not sure if this would be best to use on your specimen though because there are so many individuals in the one block. Also, these acids are considered hazardous waste, so they will need to be disposed of properly, and that will vary depending on your location.