A jailbreak server I used to regular on back in the early 2010's used a plugin that changed everyone's base HP to 200 (before item set modifications) and their default speed to 100% (also before item set modifications) and it created a more balanced playing ground for everyone, especially in FF minigames and races. I think it would be a good addition to the flux.tf server, and would also help to make classes not normally played more versatile (like spy and scout).
P.S. REMOVE GUARD CRITS
I see where you are coming from, but I've seen every class played and having their niches, besides really engineer which is played for aesthetic reasons.
Especially scout and spy, who both move faster than usual, the former being 33% faster and having melee projectiles, quicker melee, and the Boston Basher, and the latter having an insta-kill that is so easy to abuse that many jailbreak servers have an anti-backstab feature for the warden. No one is forced to pick a class, it's kind of silly to imply otherwise, as if they are forced to play Heavy and be slow. Everyone could stack soldier with banners and Half-Zatoichi/Whip. Still, we tend to see most classes played, some more than others, but it's not like any class is unplayed.
I guarantee you that the way it is now allows more class diversity. How do I know? Right now, every class, even the good ones like Soldier and Medic, have obvious downsides (Soldier is slower than every class besides Heavy, and requires a teammate with him to make him slightly faster than Medic/Spy). Remove everything about a class, and now you just have choices of weapons, to equip on a 200HP, 300HU/s speed class. And also, I have yet to see a Jailbreak map with an obby or mini-game that is impossible or near impossible for a Heavy to participate in and win, so that is a non-issue.
Weapons like the Warrior's Spirit and Fists of Steel have now lost their value on Heavy. The Half-Zatoichi on Demoman is worthless compared to using it on Soldier with a banner. You are also buffing the strongest classes, with the Medic now having more HP and his healing targets are guaranteed to have potentially 300 HP. Soldier is able to move faster than a base Scout with the whip. Sniper is literally one of the best classes on many maps with sightlines and spots for ammo to kill warden, and you are giving him 75 extra HP. Same with Spy, giving him 75 HP is a hard buff to one of the best rebelling classes to kill the warden—imagine the nightmare of a 300 HP pocketed spy trying to trickstab you as warden. Engineer will be as useless as he was. You can see the problems here.
This was not to say that you had a bad idea, because 100% there are reasons to consider this change, reasons like people wanting to play the class they want to play because they have hats for that class or they find them aesthetically pleasing, but dislike the gameplay or limitations of that class, and want the gameplay of one class with the hats of another, among other reasons.
Also, the guards having crits is just a server-sync issue, it will be fixed as soon as possible. It is not going to be changed and not up to debate at the moment, as most players have made it clear that they prefer the 50% damage boost instead of crits.