I'm sorry, but I gotta throw down a big fat NO on faster spawning for mini-bosses. Having a meatshield with target points like the head of a Panzer Soldat or orbs on a Thrasher requires technique and concentration sure, but ramping down the difficulty just to have them appear more alongside regular zombies and whatever extra mini threat (spiders, dogs, etc) just creates an unnecessary clusterfuck.
Nothing feels worse than dealing with regular zombies while being aware of what's near your foot and attempting to remember where that big guy was last you saw him. There needs to be separation between each unique threat so you can strategize instead of unloading everything you got into a horde of different enemies in hopes to wipe 'em out.
The real challenge is knowing what may come next and worrying if your current loadout is able to handle it.