You can play any player in GK position without any problem, last season i played my DC on GK spot, because my 2 GK got injured in same day. I won the game and he had a 7 in rating.
He played the next 5 games, always with good ratings.
2 seasons ago i also played my sub GK as DMC because i dindt have anyone else for the place, i won the game, he scored a goal and got a 8 in rating.
Only If you dont have 3 defenders
1 DMC or MC
1 player on each flank but no more then 2
At least 4 player on opposite midfield
At least 1 ST
You Will have only 20% bonus possession, meaning you are playing ilegal formation.