How would you do deal with say all the subs from both sides steaming on to the field of play (could well happen)
If you dish out a yellow for original foul, yellow each for the players who have started the mass con, then 7/8 yellows for the players entering the FOP
You'll be told you've lost control of the game.
First of all, being told you've lost control of the game is par for the course. You haven't lost control of the game; the players have lost control of themselves. Deal with incidents appropriately and intelligently and you won't go wrong--even if the players do.
Now, in terms of the substitutes streaming onto the field during a mass con, it is kind of an established and unspoken agreement that Mass Cons are a different breed. It would be foolish to caution every substitute who entered the FOP without your permission just as it would be foolish to caution every player who left the FOP during the confrontation. You have the option of cautioning the first substitute from each team who enters the field
if you think it's necessary. I don't think it is.
In the case of substitutes entering the FOP during a Mass Con: look for the players who come out and
add to the confrontation by creating trouble. Some subs or even team officials will enter the FOP to try and restore order--these are your friends and don't deserve punishment (they really deserve thanks).
Punish misconduct which is violent, punish the instigator and retaliator, and get the game rolling again. Don't forget to adjust your tolerance level immediately following Mass Con to be
very strict with players--we aren't playing any advantages which aren't clear promising attacks, we aren't letting borderline tackles go. This doesn't mean making up offences, it just means that those 2/10 tackles are now definitely fouls.