But the throwing team haven't declared they're having a shortened LO. Would you ping the non-throwing team for having too many players if they had 3 players and the throwing team only had 2? Unless they state otherwise, I'll make them wait every time for the LO to be properly formed. Unless they take a QTI, but this isn't permissible under these circumstances.
This is covered in Law 18.14:
[FONT=fs_blakeregular]Unless the throw is taken as soon as the lineout is formed, the non-throwing team may not have more players (but may have fewer players) in the lineout than the throwing team. [/FONT][FONT=fs_blakeregular]Sanction: [/FONT][FONT=fs_blakeregular]Free-kick.[/FONT]