Green can get flying on rare occasions. As this is a flyer to kill fliers I guess that works.

Swarm is... wow. The prospect of "Ok, I reach 8 mana. Drop a big bomb. Think you can deal with it? Drop my entire hand." is very very enticing.

The ability to copy other flyers I'm not so sure about. This would be a 10-cost kill spell; so it's likely to end up having to trade with their mighty dragon (or, worse, chump it)

So it's more likely used to kill their small blocking flyers to that it can stomp through. Which is going to make this irritatingly small... Still, it looks interesting at least.

One thing you have to watch out for in this kind of mechanic is "Turn 1 Nourishing Shoal pitching Primeval Titan, free 6/5."

Heh, yeah, that is unfortunate. This would be active even if you cast Scornful Egotist and demorphed him. It doesn't matter how much you payed for the creature... it only wants to know what the creature's CMC is right now.

This might not be a problem if you play the "I don't really care about Legacy and Modern" card. Zenkdikar had traps... but you probably deserve to drop Swarm on the table if you got it there using a trap.

Scornful Egotist doesn't work: you can't morph things face up while they're still on the stack, and the CMC of the spell you played was 0, even if it becomes 8 after you let it resolve and morph it up manually.

Ah, I'm confusing 'spell' with 'permanent'. Okay, you're right. It won't count what things are now, but what you once paid for them. Well, that's not really a problem, then. Most alternative cost spells make things cheaper, not more expensive. Besides the shoals, I have a hard time imagining something else that would break this mechanic.

Apart from that, there's also Walker of the Grove letting you cast this on turn 5; or Avatar of Might getting it on perhaps turn 4 after a Hunted Phantasm. Neither of which is anywhere near as clever as SadisticMystic's turn 1 option. Which is vaguely remniscent of the wacky Modern combo decks which use Blazing Shoal plus Progenitus to attack for 11 flying infect on turn 2 with an Inkmoth Nexus. It's an awesome 3-card combo, but it is still a 3-card combo. And you can't even use Summoner's Pact to get the missing bit with this card because you don't win the turn you drop it. So it's kinda breakable, but not really broken.

Are we sure Walker of the Grove and Avatar of Might cost 8 while being cast? It seems to me that if the X spells change their cost when cast, then cards that say "This costs less to play" would get the same treatment...

Nothing ever changes a converted mana cost except transforming, turning face up or face down, becoming a copy of another object, or setting an X value while on the stack. Paying 2 for Avatar of Might doesn't change that its converted mana cost is still 8, just like you can pay 3 for a morph creature but its converted mana cost as long as it's face down is still 0.