A small aesthetic trick: your livestock will follow the shephard around if the shephard whistles to them, so if you can make the shephard go somewhere while livestock are following then you can get the animals to follow.
One of the simplest ways to do that is to create a small pen on one side of the square, and let it fill up with poyos; then get all your other hearthlings into position (e.g. at mealtimes, or give them a quick job which has openings for lots of them so they’re idling in that location). Once you’ve got all the other “actors” in place, your stage is set… pause the game, remove the poyo pen, and place a slightly smaller pen on the other side of the market. The shephard will call the poyos over, and they’ll all follow at first. But, as they’re crossing the square, the whistle effect will wear off, and some will fall behind while only the ones destined for the new pen will keep up.
So, you should have a few moments where there’s a line of poyos following your shephard, and more poyos idling around in your market square.
Obviously this only works for a short time, so it’s good for screenshots or if you want to set up a small scene for a larger video.
Going back to the original issue: I agree that we should be able to place animal pens on hard ground like stone or constructed surfaces. However, I think there should also be an option for natural pasture, which gives alternate benefits since you need to have that area of grass present. So, that gives a choice: the player can have smaller, more efficient animal pens but need to supply feed; or have larger and more productive pastures which keep the animals happier and give some kind of bonuses (faster productivity, faster breeding, or something like that).