Just a quick heads up before I go into more detail, whilst I think this is probably possible without, I would really recommend investing some time and resources in the FOB mechanic as it makes it a tonne easier.
For this achievement all teams must be at Lvl 50 or above at the same time. The Team's maximum capacities can be increased through building more platforms for that team (up to a maximum of 4 per team per base) or building more Command Platforms (raises all teams maximum capacities albeit by a smaller margin and again a maximum of 4 Command Platforms can be built per base). The teams are as follows.
Research & Development (R&D)
You want to be extracting staff right the way through this game with the focus being most on R&D and Base Development I would suggest as you will be ahead of the curve in terms of equipment you can deploy with and have a nice, constant stream of materials running in. Bear in mind though that you won't be getting the really high end staff members until later in the game as when you first start out you'll be finding recruits with C grade stats if your'e lucky.
Essentially this one is a long haul. As mentioned above you should be Fultoning as many vaguely useful recruits as possible as higher level teams really do make the game easier and more enjoyable.
Like many people I have seen talk about this game, I completely ignored the FOB missions to begin with (not least because the servers were buggered at launch) but after completing all the missions and maxing out my Mother Base with most teams between about LVL 47 and 51 and the dreaded security team at a measly 34 I thought I needed to do something drastic.
Just to explain things regarding the Security team a little better, as far as I know at the time of writing this is how it works.
The Security Team is the last team that you will unlock, it does not have it's own Platform and thus you can't raise team capacity by building additional platforms, as far as I'm aware, Security Team size is dictated by your number of Command Platforms and recruitment to the Security Team is dictated by the Combat Grade. However, higher level Combat recruits will be put into the Combat Unit automatically and the less skilled recruits who can't fit in the Combat Unit go to the Security Unit.
Of course, you can manually move staff around, which I forgot about a very short way in to the campaign as the game does a good job of putting guys where they should be but could potentially make this a little easier, provided you have the manpower to pull guys away from other teams to bring the Security Team up while maintaining the others above 50 too.
In the postgame you can at times find 3 out of 4 or 5 S ranked soldiers per Guard Post when you're lucky so don't fret if you're playing and keep getting poor staff members as they do scale with progression.
Hope this helps at all if anyone is unsure on the requirements and if anyone has anything to add please comment and i'll add in any useful information.