Theoretical Legal Formation Requirements:


Goalkeeper (Must be player's natural position) <---- Don't think so... see earlier screenies
Some combination of at least 3 defenders (i.e. 3 players anywhere in the back row )
At least 1, but no more than 2, players on EACH outside flank. {the D(L/R), DM(L/R), M(L/R), AM(L/R) slots} <--- watched this happen in-game.... I'd say verified true!
at least 3 midfielders (DM*, M*, AM*) at least one of which has to be in the middle row (MC, ML or MR) *This rule isn't totally confirmed
At least 4 players on the opponents side of the pitch. *This rule isn't totally confirmed <--- I have personally suffered under this rule for 28 painful game minutes while trying to play ultra-defensive!
at least 1 Striker


Just by way of updating and/or adding to the discussion....