My main point being is that all the states don't necessarily use RCT style ratios when launching assaults and regulate their armor to mainly garrison duty.
Actually a lot of the older fluff stories will not that XYZ mech regiment attached with 3 support regiments or 10 support regiments.
The LCAF has a lot of notations like that, or 3 Mech Regiments supported by 30 Conventional Regiments, etc etc.
They used them, they just didn't have them permanently attached & cross-trained like the FS RCT does.
On the "five mech to tank" ratio though there is one thing. I'm pretty sure the five-one ratio of armor to mechs you are referring to comes from FM Fed Suns although this number is a little deceptive. FM Federated Suns divides the AFFS into four parts, the Regular Army (RCTs and independent line regiments), the Militia (planetary and march militias), Training Units, and Mercenaries. According to the book the five to one number refers directly to the ratio of mechs to armor in the Regular Army and not to the entire AFFS. Now apparently planetary militias are not part of the AFFS but March Militias are as they are included in the FM unlike the planetary militias.
Actually the "over 5-1" comes from TRO 3067 vehicle section intro IIRC.
March Militias are part of the "Militias" but because they are full Mech Regiments assigned to a Region they get covered in the text w/ all other mech units.
AFFS in FM:FS has about 50 Mech, 250 Armor, and 300 Infantry regiments in the Regular Army, 30 Mech, 60 Armor, and 150 Infantry regiments in the Militia, 10 Mech, 5 Armor, and 10 Infantry in Training Units, and 20 Mech, 10 Armor, and 20 Infantry in Mercenary Units. This gives us a total of about 110 Battlemech, 325 Armor, and 790 Infantry Regiments for the AFFS or roughly ten conventional regiments per mech regiment. This ratio matches that given in the Davion House Book that states that the AFFS has in 3025 118 Mech Regiments and a little over ten times that number in conventional units.
Your going to be off a bit there.
Because Infantry is 2/3 of all military forces in the AFFS
So total the Mech, ASF, & Tanks, then Double that for Infantry.
I go with the following.
Per Mech "Marker/Unit"
Tanks are Over 5 but less than 6 times as common as Mechs, so 5.33-5.5
ASF Are less common than Mechs but the only ratio that I have ever found is that the FWL produces about 2 Fighters for every 3 Mechs. So call it 0.5-0.67 ASF to account for less ease of repair.
Collectively its about 6 Vee/ASF to 1 Mech.
So 7 is the Non-Infantry total.
Double that for 14 more in Infantry.
If we put that into AFFS RCT terms you get the following.
1 Mech Regiment (RCT)
3.33 Armor/Artillery Regiments
5 Infantry Regiments
2 Fighter Wings
2 Armor Regiments: (See Below)
9 Infantry Regiments (Grouped into a pair of brigades of 4-5 Infantry Regiments & 1 Tank Regiment
2 Fighter Wings: (Assigned to Fleet Command Groups or Planetary System Defense.)
Now account for roughly 50% of all Mech Regiments not being RCTs but still having that amount of Fighters, Vehicles, & Infantry to account for.
You start to see just where all those (3 Mech Regiments + 30 Conventional Regiments) lines come from in the books, or, where those lines about unattached fighter wings/regiments come in.
There are plenty of conventional brigades to go around to have some on offense supporting invasions while others are stiffening up defense on major worlds that rate more than just PUG/Militia for defense.
The FS RCT just takes a decent amount of support troops & trains them to operate closely together so that you get better tactical coordination when assaulting a planet in force.
You'll still likely have those conventional support brigades following on to assist in conquest & assimilation of the populace, but the RCT gives you a decent starting force trained to work side by side with big walking giants that might accidentally step on someone that isn't used to being around them.