I'm going to check things out. I think if I'm right, I can prevent all slime spawning on the bottom floor by putting down something on top of the hoppers, like string, redstone, or carpets, to prevent the game choosing it as a pack spawning location, and the water will take care of the rest of the pack spawning for the slimes on the bottom floor and nothing will spawn down there. And I'll go double check the lighting in the farm and get back to you, and let you know if the slime farm is working as intended before you go ahead and build the entire thing.
So this is the spawnin pad size / design. You will have quite a few of these when you're done There is 17 dispensers on the long sides, and from each dispenser it comes out 8 more blocks, which will make it 35 long and 19 wide, and then fill in the edges like so. This is in order to allow you to turn off the farm with water streams.
Like so. Each layer is stackable and there will be another floor every 5th block, so with 4.5 air blocks in between floors.
You'll have to build this from the bottom up though so you need to plan out where the first spawn pad's Y level will be. I usually make the floor level of the farm one block above bedrock, and then make my first spawning pad on the 4th block up from there, because you will need to have enough space for iron golems under the lowest spawn pad to kill the slimes. So if you're using slabs like I did, you don't have to though, it should have a 3.5 block air space between the bottom of the farm and the first spawning pad.
When you figure out the what Y levels you will start on, next you'll need to figure out the X / Z positions. Find the center point between the two slime chunks and that's location of where the piston's from the picture will be, and the center point of the spawning pad. I'd mark that location out. Since it's an even number though and there is no true center point block between the two chunks, just pick one on either border of the chunk, doesn't matter which one.
Next it's time to dig out a huge area. I'd say from Y=45 down to bedrock you'll need to dig out. From the edges of the spawning platforms you need to dig out 3 off the edge of the corners of the hexagon, and 2 more out to the side from there (as shown in the picture) in order to allow for enough space for the big slimes to fall down. For the area you dig out, the long side is 41 long and the wide side is 25 wide, and again the edges to form a hexagon. Then you'll need to build your spawning pads.
Now for under the spawning pads. There's a few things to note here. First the water streams. In order to keep them from merging and becoming a source blocks, you will need to place them at head height around the entire bottom area of the farm. Where the water streams end are hoppers, to collect everything. You can't slab the hoppers unfortunately, so make sure to light that area up well. You don't want hostile mobs to spawn down there and kill the golems.
Then with the hoppers you can just direct the collection hoppers into a line of hoppers in the floor that leads outside of the farm, and create a storage system from there.
Next thing to note, above the water streams you will see I have a solid block, sand, and cacti. That's how this is also a cacti farm, but there's another reason for it. Mob behavior changed a little while back and now hostile mobs will try to attack golems without the golem provoking them first. I have found with slimes, that only the large slimes can actually hurt an iron golem, and medium and small ones are harmless. So I lined the outside of the bottom of the farm with the cacti, which creates a bottle neck for the large slimes, and they won't be able to make it to the bottom until they die from the cacti and split up into medium slimes.
You will need to dig out behind the cacti in order to plant them, they need to have air blocks on all sides, so poke holes in the wall like so.
Whenever they grow, they will pop off because of the wall of the farm being there, and the cacti goes down into the water streams.
And finally for the actual on / off mechanism.
Pretty simple, Behind all the dispensers is a solid block with redstone dust on top of them, it doesn't need to be a redstone lamp, but be sure that it's still bright enough when you cover the glowstone in the spawning platform that hostile mobs don't spawn. Under the center solid block + redstone will be a piston with a redstone block so you can send the power downwards to activate each floor underneath. I didn't build the other floors, but you can see where where the slabs are I made around the other pistons are where the next floor up will start.
Which should look like this when it's activated.
And then from the very top piston, lead a signal from wherever you want to turn the farm on / off from, and you can turn it on / off easily with just a click of a button.
Whew, that took longer than I expected. Sorry if it was a bit long, I wanted to be thorough. If you have any questions, let me know.
Thanks a bunch for putting in all the effort man. To be honest, I'll probably build a faster-to-build slime farm for now, and when I get better tools (and a beacon!) i'll definitely be upgrading to this one (simply don't have the supplies for now.. haha).
I apologize for the lack of updates, life's been in the way. Semester exams are coming up in 2 weeks too, so that's not too good. Hopefully I'm able to get at least one update before then though.