Finally I've been able to test this out.
Bug report - I can't seem to get it to work :D
This is my json:
[
{
"techguns:slimy" : {
"chance": 50,
"entities": [
"techguns:alienbug",
"techguns:alienbug",
"techguns:alienbug",
"techguns:alienbug",
"minecraft:spider"...