Having someone with some power to help sweep up isn’t a bad shout, but it doesn’t have to be a CB, a DM or powerful midfielder who doesn’t mind sitting a bit deeper is the way to go for 5 a side. Like you say though they’ve got to have some skills to back it up.
How about 5 a side team with max 1 player from any prem team? Stops people going heavy on city and Liverpool players.