I would like to take the time to go on a rant here about HOW MUCH I &%$#)&! HATE TEAM STACKING.
As anyone who's played TF2 would know, this FPS is unlike any game that's come before it in the genre. The nine classes each have their own strengths and weaknesses. The game is easy to pick up and play, but to master it, you need to take the time to learn these aspects and learn the best times to be a certain class. The game is designed for team-oriented play, there's no such thing as a one-man show. If you think skills from other FPS games will help, you're sadly mistaken. One hero is not enough. It takes at least 6.
So you can imagine my frustration when I see people constantly stack scouts, snipers and or spies to the point where we're guaranteed to lose. Each of these classes are way too frail to be stacking. They only have 125 health, and their special quirks don't make up for it. The only way to get around the health problem is practice, practice, practice. Learn to take advantage of the scout's speed to dodge and harass your attacker. Learn to predict and aim with the sniper rifle. Learn to get behind enemy lines without detection and stab key targets. All while avoiding bullets, rockets, grenades, fire, and so many other hazards that no other FPS can comprehend. You need to be sure you know what you're up against before you even THINK of playing those classes.
The same goes for MvM. There's only room for 6 players in a game, so stacking is really not a wise idea. You need a strong team composition to be able to survive. A good rule of thumb I find is to have two classes for offense, two for defense, and two for support. One of each class is enough to take on the bots, as long as you know which class to be.
Soldiers and demos are fantastic for clearing out groups of bots, with their explosive damage wreaking havoc. With a kritz medic or crit canteen, no bot can last very long.
Heavies are great for mowing down bots one by one, and having knockback rage on the minigun can be a real asset to the team, especially with giants.
Engies are a must have on any map. Their teles can get people to the front lines in a snap after they respawn, saving time and giving the team a better chance of victory. Dispensers for teammates and sentries to catch any strays are also vital for effective support
ONE SCOUT IS MOST DEFINITELY ENOUGH. With the ability to suck up money from the field, the scout's main priority should be collecting the cash your team needs to upgrade. It makes it easier to earn bonuses which can be really helpful. And since money he grabs gives the scout health, he can keep at it for virtually an eternity. That being said, having two scouts can be problematic as the health bonus would be going all over the place and cause huge problems for the other scout.
I would rely more on medics than spies or snipers in this mode, 'cause medics are very good keeping the team alive and helping output lots of damage. Spies are only good if they know to use their sapper and backstab as many key targets as possible. Snipers are only good if they can aim well.
I don't care if it's on a pub server, it's never fun to get rolled by the enemy team when people are just screwing around and playing a class they shouldn't be. Unless it's a challenge issued by frequent visitors or donors, then it's fine as long as you play nice. Stacking heavies with gloves or mittens can be quite enjoyable, but is easily ruined by one party pooper that goes as a spy. Those types of games are fun, but in a normal game it's pointless and gets your team nowhere.
And one more thing: if you're gonna be trolling the player who's asking people if they could switch to a class we need, you need to uninstall the game. There comes a time where you need to get put your game face on and teach the other team a lesson. And the only way you're gonna do that is look at the roster and see if there's another class your team can use to win the game. If you're gonna be a gigantic knucklehead, go back to Call of Duty where that stupidity belongs.