When my team played 7v7 we couldn’t defend for toffee, but we could score goals.
But was irrelevant if you were conceding more than you are scoring!
I tried three at the back, terrible, two at the back, terrible, got smashed in a game, kicked off with the parents cos of how bad it was (couple of bad eggs mainly).
I thought I’m going to throw caution to the wind and just go for it, and played the best defender at the back, on his own. It worked a treat!
We never really looked back after that.
My advice would be, to try things out in friendly matches, different formations, players in different positions etc.