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GDI= the Orca
Brotherhood of NOD= the Apache
You need to "remove AI player" from the NOD side then join team two.
On campaign you can only play as NOD, but online and skirmish they are playable.
No. Kane's Wrath only has a Nod campaign.
CNC3 Tiberium Wars has a 4-mission Scrin campaign, but to unlock that, you must beat the GDI and Nod campaigns.
You have to finish both the gdi and nod campaign which will reveal the secret scrin campaign.