Not that Wiki is always correct but here's what it states. Guess it is legal, just odd.
The formation must have at least 7 players on the
line of scrimmage[1]. The 7 players are not required to be next to each other, so they may spread out across the width of the field, but this is rare; most offenses place at least 5 players together in a continuous line. Although it is rare (because it reduces the number of eligible receivers), teams may place more than 7 players on the line
The other players not on the line (usually 4 in number) may be positioned anywhere, but all must be at least 1 yard behind the 7 or more players on the line of scrimmage
[1]. The traditional saying is "7 on the line, 4 in the backfield" but this is something of a misnomer, as "backfield" usually refers to the area directly behind the offensive line. 3 of the 4 "backfield" players (i.e., the 3 not receiving the snap from center) may line up as wide receivers as long as they are behind the line of scrimmage; these are known as
slot receivers.
Of the 7 (or more) players on the line of scrimmage, all except those at either end of the line are
ineligible receivers: these players may not touch or catch a forward pass
[2] (unless it is first touched by an eligible receiver, including a deflection by a defensive player), and on a forward pass play, they may not advance
downfield (i.e., across the
neutral zone) before the pass crosses the neutral zone. Ineligible receivers may advance freely on a running play or after a pass is thrown. These players typically have uniform numbers in the range 50-79 to indicate they are ineligible.
