Source? I was under the impression that conventional support was assigned on an as-needed basis, with the precise quantity being variable.
Edited for relevant text
SLSB P.133/134
REGIMENTS
The SLDF Had 7 Different kinds of "Battlemech" Regiments
Line Regiments x3 (HeavyAssault, Battle, Striker)
Made up Battlemech Brigades
Independent Battlemech Regiments x4 (Dragoon, Hussar, Light Horse, CAAN)
Operated Solo or supported Infantry Divisions to boost mech #s
In keeping with their independent nature, these Mech regiments had companies of non-mech units.
Jump Infantry, Hovercraft, Tanks, & Fighters (Also some Watercraft for CAAN)
When you also toss in the fluff about RCT's being made up for 4 Mech Regiments, often Independent, & the fluff about the Colossus supporting RCT's, I personally like to think of the Colossus as being about the most you should attach to an Independent Regiment.
Which still gives you 72 Vehicles & 12 Infantry Platoons. Less if you suck off some of that space for Security, Techs, Medics, Cooks, etc etc.
But easily could be 2-3 companies of true Combat Infantry & 3-4 Companies of Combat Vehicles all in support of your 3 Battalions of Mechs.
I don't know about "as needed basis", I've never read that.
I'm also not saying it doesn't vary from Regiment to Regiment, it could.
Just saying an entire Regiment of a single support type pushes the bounds of calling it a single "Independent Regiment"