1) I don't understand what you mean by that. This game is rated Teen, so anything within those parameters should be acceptable.
2) Dark Blast and Dark Melee are probably okay, but Dark Misama is just a very powerful set period. If you give many of your custom foes powers from Dark Miasma, do so very carefully (i.e. only give 1 or 2 powers to each critter, and never give tar patch or darkest night to minions), otherwise they'll just be frustrating to fight.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
Well, I saw "Kid-Friendly" as one of the tags, so I wondered what would qualify an arc for that. And what arc would not qualify for that tag, but still not hit the threshold for Inappropriate Content.
In a mission where all the foes have Dark Blast or Dark Melee, limit Dark Miasma to Bosses only or skip altogether?
Stuff I would definitely NOT consider kid-friendly, but that is in no way inappropriate:
A convoluted Malta arc that requires extensive knowledge of game canon and makes references to real-world political extremism. An arc based around the CoT's habit of stealing people's bodies and locking their souls away in crystals. An arc full of riddles or metaphor, something that uses a heavily non-linear narrative, something with a large amount of text that must be read to understand the plot. Humor based around in-jokes, forum references, internet memes, or something else a kid wouldn't be likely to be exposed to.
A kid-friendly arc should be done in a way that a kid would both easily understand and be engaged by. I don't consider "kid-friendly" to mean "teen friendly," the game's rating covers "teen-friendly." I consider "kid-friendly" to mean "suitable for pre-teens...if they're old enough to play CoH, they're old enough to play, understand, and enjoy this arc,"
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
Arc ID#431270 Until the End of the World
Ah, I see. That makes sense. Thank you. 
1) What makes an arc qualify for the "kid-friendly" tag? A failure to add anything that would be considered offensive material?
Not getting too graphic about the form of "defeating" foes you will have?
No top-heavy, bikini-clad custom critters spouting lines from ERP?
2) Would a custom group that uses various Dark powers (a mix of Dark Blast, Dark Melee, Dark Miasma, Etc. in the group's foes) be too obnoxious in the level of ToHit DeBuffs upon the players? Assume a L30-50 or L40-50 arc.