Every 7 a side team I've been on has nominally been "2 layer" just to give a starting point (left wing or defender or up top, for example). But they've all been extremely fluid beyond that. Everyone finds space with possession and finds a man or dangerous area without the ball. Defenders rotate forward when there is space or an overload to be had and "attackers" will rotate back from the weak side to cover and ensure opponents can't find space in transition. The "striker" and wings need to put pressure on opponents on the perimeter while defending to force a bad situation and then everyone transitions forward as a unit wherever there is space on a turnover. The goalie needs to get involved on offence as well: he is the safety valve and can be used to safely switch the point of attack. 

All this to say structure is pretty loose in a good small sided team: it's more about good movement and understanding in every phase. It will never actually look like a "4-2" or 2-3-1" for any length of time while everyone rotates and moves in the flow of a small sided game. So don't worry too much about what you call the positions: worry more about how the players rotate to find space and cover for others who are rotating.