Fern is correct.
Nothing can be on the edge tile except path or the game will crash.
I leave two tiles from the edge outside the zoo wall, placing the zoo wall footprint on the second tile from the edge, which leaves me the second tile from the edge to decorate outside the zoo wall on top of the footprint, while leaving the max amount of space inside the zoo wall to build.
Putting the zoo wall footprint on the outside of the zoo prevents accidental deletion while building.
You could put the zoo wall footprint on the second tile with the wall on the outer side, but it may cause problems (I am unsure), and leaves no room to decorate outside the zoo wall, and can be accidentally deleted while building even after the zoo gate is in because it is inside the zoo wall.
You could also build without a zoo wall, but you have no choice where your entrance is, it must be where it was when you opened the map, and you must not place anything on the edge tile.
The dlupdate folder can hold 300 files, any more than that crashes the game.
Each in game menu can hold 3000 items, any more than that crashes the game.
So you could have 300 combined files with 100 items each, or 100 combined files with 300 items each.
I have never combined files, but I will have to learn soon because I need my necessary items, (like Genkies Grasses and Tall Grasses and every flower Brandi ever made), but there are tutorials here, and people who can help teach you how to do it.
The current zoo is crashing, which means its file may actually be corrupted, you may want to just delete it and start over following the above parameters.
Hope this helps :)