I'm working on a campaign for my current table as just a break. So here's the presentation... the Grey cloaks are the imperial equivalent of the old school French Foreign legion. We don't care if you were rogue lifeing it up or a minor noble running away from getting merced by your family to make sure the "proper" person takes over. You come to the unit, give us a name we send you through our bootcamp and then send you out in squads to the border. My game mechanics question is since just about all of my players multi class anyway should the requirements be mandatory one class ranger or give them a choice? 

And I was thinking of running it as forth level just to give them a chance at getting at least some of there benefits of their desired class. Thoughts?

I don't see why you have to as just about any class has a subclass that can fulfill the role of a border legionnaire. Are you trying to "flavor" the bootcamp? Because a shared background like Soldier or something custom that they all have to take to signify the leaving of their old life behind. 

I really don’t recommend class-enforcement. Unless the party loves the idea of an all-ranger party, (and if they do, that’s great!) then I recommend explaining the theme of the game to them and letting them design characters that fit that theme.

The following fit a border legionnaires style: most barbarians (they’re the wild men in the forest who joined the Grey Cloaks), clerics of war, law, nature, and probably some I’m missing, most druids, most fighters, monks can with some subclasses, paladins definitely fit into just about any military force, rangers (obviously), rogues can, especially scouts, and the three casters can always find a way to fit in, because they’re quite versatile.

And that is only mechanically! With your power as a GM and the players’ creativity, you can easily flavor anything to work! I really must stress that pigeonholing your players into a class, particularly (and I say this with ranger being my favorite class) a weaker one (that is definitely an acquired taste) is not very fun for many people, and it will probably wreak havoc on your party composition and power level.

Utilize backgrounds and subclasses to build a cohesive group, not the class, which, in D&D at least, is the most personal and vital choice that a player makes in a roleplaying game.

And if they all love the idea, then disregard all of this! Keep in mind though, and inform them, of the drawbacks of playing a game with an all-ranger or all-ranger multiclass party.

None of that description says "everyone must be rangers" to me. This kind of campaign is a great way to illustrate how little class needs to reflect who you are and what you do. It's just how you do it that's different. I would never run an "everyone is X" campaign unless I floated it with the players first and they all got excited about the idea.

Quote from Brune_Mckown >>

should the requirements be mandatory one class ranger or give them a choice? 

 Scatter, I think this states that it is everyone must be rangers, but the question is if they should be allowed to multiclass or be “mandatory one class” rangers. I’m just trying to ask more questions and provide a comprehensive look at what class choice will mean in a game like this. Otherwise, I totally agree with you and I think it should become more commonplace that people believe that class isn’t as important! It’s about the character, not the mechanics!

if you're requiring ranger due to their army training, consider instead just giving everyone a free "made a man out of you" homebrew feat: proficiency in medium armor, short sword, and light crossbow... or whatever very standardized weapons the army hands out. they can acquire other stuff off corpses later. a feat like that reflects shared experience, martial training, and (most importantly) doesn't quash their secret identity noble/wizard/rogue beginnings or their emergent sorcerous/paladin/warlock powers. that stuff is too good for drama and character development to just leave it on the table! let players pick their class and background, but ask them to keep it secret. whatever you were before, you're a soldier now!

if you have time, go look up some Malazan Book of the Fallen (specifically, the Bridgeburners) or Black Company books for inspiration with regards to combined arms units against magic foes and occasionally with unreliable/sketchy magic support.