Here I'll list the Digimon that you can unlock and some of the Digimon's evolutions.


Digimon:

BlackAgumon:
To unlock him, defeat him in single player mode when he surprisingly attacks you.
He can digivolve into BackGreymon and BlackWarGreymon.

BlackGabumon:
To unlock him, defeat him in single player mode when he attacks you.
He can digivolve into BlackGarurumon and BlackMetalGarurumon.

BlackGuilmon:
To unlock him, defeat him in single player mode when he attacks you.
He can digivolve into BlackGrowlmon and ChaosGallantmon.

Duskmon:
To unlock him, Defeat him in single player mode. He is the boss 2nd to the top.
Cannot digivolve.

Diaboramon:
To unlock him, Defeat him in single player mode. He is the boss 4th to the top.
Cannot digivolve.

Omnimon:
To unlock him, Defeat him in single player mode. He is the boss 3rd to the top.
Cannot digivolve.

MaloMyotismon:
To unlock him, Defeat him in single player mode. He is the boss on the top.
Cannot digivolve.

Neemon:
Yes, you can fight with Neemon. He has some funny attacks too. To unlock him, Defeat him in sigle player mode. He is the boss 5th to the top.
Cannot digivolve.

Stages:

Ice Palace:
In single player mode, go to the boss on the bottom, Defeat them and you unlock it.

Chaos Wasteland:
In single player mode, defeat any of these digimon and you unlock it.
Duskmon, Diaboramon, Omnimon, MaloMyotismon, Neemon.

I'll post the mini-games later on.