For future reference, any Shadow Lords related questions should be in the Shadow Lord’s sub-forum rather than general, but I’ll try and help out.
Edit: Thread has been moved. Crisis averted
Typically you’ll want to have characters that have traits that compliment each other. Taking characters with different strengths and playstyles can be quite useful.
One area to look at is their Allignment. Each character is filed under Order, Chaos, or Ultratech, and certain missions may have different results (or be unavailable entirely) based on your character’s faction.
So let’s apply this to Eyedol. Eyedol is a combination Rushdown/Zoning character that switches playstyles at random, and he is under the Chaos faction. My advice would be to pick one Order character and one Ultratech character to balance out your team, and try to find at least one character who’s got a more stable/straight forward gameplan that won’t be subject to change mid match.
Guardians are mostly just personal preference. I use my Killer Fractured Ward for everything since I like the armor effect, but others will tell you they have different favorites.