Spawners
I can look into fixing up the /spawner list to only show what's available. This thread here has all available spawner types. Skyking through Donor400 all have the same available /spawner types, so you don't need to ugprade any further to unlock anymore spawner types.

I'm not fully sure why parrots and horses aren't available immediately with /spawner, but I'd imagine it'd have to them being "luxury" mobs, in that they're generally just used for display, and don't have a real purpose. Spawn eggs for them can be purchased from /warp trade, which you can then right click on a spawner to change into the corresponding spawner type.

Blazes I prefer off, even from a non-staff perspective. Since you can't mine blaze spawners in the nether, they're always readily available for anyone to farm. Helps keep blaze rod's value stable, as blaze rods don't have many common uses on the server, and blaze spawners are very easy to automate and farm tons of blaze rods. Along with that, blazes have a bunch of moving parts and particles, which does a good bit of damage to server performance, along with player's TPS if the farm isn't managed properly.

Mob Stacking
While it can be a bit annoying to get used to, I do overall enjoy the mob stacking. It helps keep farms clean, and allows for you to stack of large numbers of a certain color of sheep and shear them all. Along with shearing sheep, you can also apply a nametag to mobs to prevent them from stacking. Name tags are relatively cheap, and can be obtained fairly easily from fishing, so I don't think getting a bunch of nametags should be too much of an issue.

If you get a spawner, this can also just spawn in the mobs for you, so you don't need to worry about breeding. Skytitan+ can also use /spawnmob, so they can spawn in a decent number of farm animals for you.