Okay, my take. Most Houses organise in terms of coherent regiments. So for example, in 3067 the 5th Galedon Rangers has it's regiment of 'Mechs, accompanied on a regular basis by the 42nd Aerospace Wing, 156th Galedon Regulars armour regiment, and the 503rd Motorized Infantry regiment.
In the field, the CO of the 5th might spin off a combined arms taskforce at company or battalion size, for a particular mission. Eg. a company each of 'Mechs and infantry to support an artillery park, or lance of light 'mechs, platoon of hovers, and infantry in fast APCs to raid enemy lines.
The Davions & Steiners formalised Regimental Combat Teams - formalised as a regiment of 'Mechs, three of armour, five of infantry, two aero wings and an artillery battalion. Following the Jihad, it was hard to maintain this level of forces, so they moved to the Light Combat Team - similarly made, but with battalions rather than regiments.
You can get an overall guide
here. Short form - for your game, your headspace, do it how you like. I have a hobby of making combined arms battalions - a company each of 'Mechs, armour, and infantry - because that's how I roll ;)
W.