Nope, that isn't possible with spigot.
As for the idea, amazing. I would love to have this in game, it would create many different possibilities. But would the game render these "new creations" as whole blocks, or just entities like itemframes?
Although a group of 20 seems a bit too much, I dont mind having a max group size of 10 or so. Even if the same mechanics of pigmen were applied, It wouldn't really matter because pigmen aren't hostile (unless you hit them) and its not like other mobs will start defending eachother like wolves and pigmen do.
I like the idea of groups, it seems like mobs would work together. However, I think this should only be for some mobs (maybe zombies, zombie pigmen, slimes, magma cubes, endermites and spiders) and that groups wouldn't necessarily have a set limit of mobs, but would be rarer. Also, some of these chances seem a bit high, like even if it has a 4% chance per second that would mean in 30 seconds they would probably become a group, and it is somewhat quick.
I am an adaptive creature! I adapt to the situation, and become a creeper that eats cats! I devour the cat's essence and evolve into the nyan cat! I teleport through interspace dimensions and blast Genius_Idiot with rainbows.
Next poster is stuck with me in space, listening to Nyan Cat song for all eternity.
Making this guy immortal would just be too much. Dogs are hard to find in certain seeds or biomes already. Considering the time it takes you to find some dogs to tame, you didn't really say it wasn't worth it. Besides, if you don't want the dragon to die then why don't you lock him underground covered with glowstone so that nothing can kill him? The Dragon is supposed to be YOUR pet, not the other way around. Take care of him, he's one of a kind.
Uh... Guys... This is the equivalent of adding a zombie pigmen mechanic to EVERY MOB IN MINECRAFT THAT WANTS YOUR BRAIN. Imagine 20 slimes. Or, for that matter, 20 anything. In a cave. With lots of spawns. Yes, I am scared now.
Lets see... 2 archers for long range, couple of endermen for some creepy attacks, then a slime tank to force you into a dead end with a horde of zombies to finish you off, with a creeper to ruin your items. Uh...
There is no definite time. This 6% doesn't increase every second. It will stay as 6%. So creepers will remain rare. 15 seconds of being near another group won't definitely add a creeper to it.
AI is smart enough to move around blocks to find a closer route to their opponent. Besides that, it's more difficult to build a large area to trap 20 zombies while moving around, mots likely while hostile mobs are spawning around you. Have you ever tried that anyway? Your better off running away then trying to fight back or build an area to trap them. Even if you did, what's the point of trapping them anyway?
When I said trap, I meant doing anything from just building up, building a minifort around yourself, or in a cramped space, and making it so you can hit the zombies, but they can't hit you. As for creepers, its the same thing. Sorry if I wasn't clear.
It is once they start getting into groups.
All that happens, is that the group would have a higher damage and more health than one mob. You can still deal with it in mainly the same way.
Who said anything about killing them? There is also running away, using a bow, or using potions.
Creepers have a 6% chance per second to join a group. Do you realise how hard and rare it would be for even a group of 2 creepers? It probably wouldn't happen unless they were in the same area for a long, long time.
No, your only option won't be to run away. You can use your brain, and outthink a bunch of AIs. See a group of 20 zombies? Use blocks and trap them. See a bunch of creepers? You probably won't, but if you do, its basically the same method. See a bunch of skeletons? Try to make them shoot each other. Is it really that hard to outsmart mobs? If the only way to kill mobs was to charge in and attempt to kill all of them with no strategy, then yes, you will die fast.
I bribe the piano to show me a secret passage out of the room. I bribe the bat to follow me and hit sylv30n in the face.
Next poster is trapped in a room with no windows or door, with a table and a mirror (It's a trick question :D)