As to whether CC#18 would add flutter to an instrument in GPO5, I haven’t a clue and it would be weeks till I checked it out. I suspect not but could easily be wrong.
At this point, I’m bowing out of this topic. Hopefully I’ve pointed you in the right direction. There’s a similar MM users board for Garritan Instruments. You’re likely to get better answers there regarding how to get the most from these libraries.