Need some advice!
How many mystic knights should I keep of each color? I have heard 3 green, 2 black. I am unsure about red, blue, and white, though. I could clear out a lot of space if I had a good idea about which I could feed.
Keep one of each. Even with the Green/black, I'd say two of each of those (though 3 is the "ideal" team for green).
The ADK (Green) team is ADK Lead, ADK ADK CDK (black) CDK, ADK ally. This gives you a shit load of orb changing, so you can generally ignore the other orbs. I don't have the team, and I'm unsure it would be all THAT great. I run ADK, Inferno dragon, Black ripper, White ripper, Blue ripper, ADK ally. I only use my ADK for farming with a SMALL bit in other teams.
The value behind the mystic knights are that they have an orb-changing skill. So in mono teams it's a great slot to help power the team. Mystic Light/Verche (Dark to Light) is essential in any of the big Mono-light along with Fuma Kotaro (Wood to Light), Valk (Heart to Light), and/or Apollo (Dark+ Heal to Light).
There are 3 "different" types of orb changers: Opposite, Off, and Heart. Opposite is the Mystic Knights, off are the new ninjas, and heart are the Vamp/Valk/Zerk/Highlander/Titan group. They all have various leader abilities, but most are focused for on their orb changing.
Sorry, random, long, etc.
TLDR version: Keep at least one of each, two of the Green/Black if you feel like you want an ADK-dragon team.