@Start-End:

Tottally agree with you.

@OP:

I feel some mini bosses should appear randomly, while others are scenario bassed.

On the plus side, Compounds are the only places where bombers can really shine.

On the down side, I'm not a bomber.

Minibosses showing up would be pretty neat. Technically trojans and giant lichen colonies are sorts of "minibosses", and are often placed in levels to be a challenge.

Creating a few minibosses exclusive to the arcade would be neat. I would say give them specific spawn scenarios though, which can be placed in random generation (imagine finding a tortodrone in a clockworks tunnel, or a giant toxigel in a deconstruction zone).
The difficulty of the levels doesn't mean a miniboss can't spawn, all that needs to happen is have a level decide where a miniboss can spawn, then choose from the most correct "pool" of minibosses. Maybe a compound won't have a miniboss spawner in it... I wouldn't be against them having minibosses in there though.

Compounds are perfect for difficulty, all they need is a better payout, especially in boxes. Candlestick keeps are great in their difficulty and payout. I can even see some interesting miniboss mechanics with the candles in there!

So if we had a miniboss per monster family, here's how I'd recommend them to work:

• Slime - Fearsome lichen colony. A larger giant lichen colony that can spawn various kinds of slimes, such as jelly cubes, ice cubes, oilers, quicksilvers, and even toxigels.
• Gremlin - Gremlin hellfire cannon. Essentially a giant mortifire, the size of a gremlin incinerator. Has added attacks and cannot drop their weapon.
• Construct - Tortoclone. The remains of a tortodrone stitched together and given mechanical life. Has construct defenses and toned down attacks.
• Beast - Snarbocub. A snarbolax cub that hasn't mastered its armor plates quite yet. Takes reduced damage from the front, and normal damage from the back.
• Undead - Kat Kaptian. A large, grimalkin-esk spookat whose fight is similar to margel, but toned down. Instead of black kats, there are spookats, statikats, and blogatos. They leave no trail of poison or spawn cursed, knight-seeking orbs.
• Fiend - Ur General. A trojan with a few added attacks, and the ability to shield two sides of themselves at once (the other two sides are always vulnerable, however, unlike a typical trojan).

These are mostly status-neutral, so they can really work wherever. They are also rather quickly thought up, so many things could be changed!
Luckily, many of these minibosses can be fought in spirit without the arcade - slime and gremlin are inspired by compound 42, tortodrones have their own event, beasts have a boss based on them, undead have a harder version as an event boss, and fiends have trojans spawning around everywhere anyways.
I would say put these in their own sort of forced "danger room", and have them placed randomly into the clockworks generation. Would make arcades a whole lot of fun for sure!