Guinea pigs need good air circulation so it may not be the best idea to do the entire cage with those. You can try the windows from this forum's store to see if they help, and put them in the trouble areas. I used the smaller ones to stop my boys from chewing on the divider.
Kicking poop or hay out of the cage shouldn't be a problem with coroplast and grids. A stray poop may be flung out, but it shouldn't cause a serious cleanliness concern.
I agree with @4boipigs that the plastic panels do not have sufficient airflow. When I made my first C&C cage I used these coroplast edging to give the coroplast a more finished look. As it turned out, the edging was slippery enough that my pigs' front paws could not stay in one position long enough to chew the bars. Since then, I was given a larger C&C cage and did not get around to put these edgings up. Now my pigs are bar chewers.
As to poops being flung out, if it is a standard 6" high coroplast tray, generally the poops don't get flung out during zoomies. For my pigs, I see their poops occasional get dropped out when it is on their front paws and they put their paws on the coroplast to bite the bars.