Most people run three or even two of certain legendary cards in their decks for this reason
Sephiroth, Fallen Hero sets your modified creatures to beefy 7/5s at base when he swings, which is fantastic if you manage to go wide
The color identity of your commander(s) still applies to cards like Arcane Signet to explicitly refer to color identity
The choice of planes includes Norn's Dominion because thats too ridiculous to ignore, some wacky phenomenon cards (we can have two of them), and planes that benefit from having small creatures, Naya creatures, or tribal interaction
however, since support for this archetype only supports monsters that cannot be Normal Summoned/Set, this is not an issue
Then, you tribute off DMoC with Cannon Soldier