To create a streak, you must combine a streak card (blue, red, green, or black) plus at least six more cards of the same color. You can add additional cards to your streak, but the minimum is seven.
You don’t lay down a Streak and Sets its one or the other.
A streak can only be downed all at once. Downing is a form of discarding.
The very first downed set or streak from any opponent is safe from being stolen until someone lays down a second set or streak.