Questions regarding DLC compatibility:
Will those with "Horde Command" pack be able to play with those who did not purchase DLC?
"Yes, players without the new content can still play with players that have it. In fact, they can even play together on the new maps if the player who purchased the Horde Command Pack hosts the game. Even though players can play together, players without the Horde Command Pack cannot build beyond the current fortification level caps."
Does the person with the Horde Command Pack have to initiate the first creation to upgrade the decoy, sentry, Silverback to its highest level?
"The player with the Horde Command Pack is the only one that can build beyond the current fortification levels, but are not required to start the building at lower levels."