Well, I tend not to acquire scenario packs, but 1714 FM: ComStar does have comments like "Level III units, directly analgous to battalions except for their use of combined arms, are the most common deployments in high-risk areas like the Clan border." This shows that, at the very least, Level IIIs are composed of mixed groups of Level IIs, though it doesn't prove that the Level IIs themselves are combined-arms.
For my money, I see plenty of advantages to having at least some Level IIs being of the same general unit type (i.e., all battlemech or all armor); it aids the age-old military dictum of concentration of force. Technically speaking, a US 4-man fire team is "combined arms", since the team leader usually has an under-barrel 40mm grenade launcher, meaning the rest of the team can use direct-fire from their rifles to keep an enemy pinned in a location where the grenadier can hit them with indirect fire -- which is what is really meant by the term "combined arms" in military circles. It certainly wouldn't be necessary to have more than two unit types in a given Level II, though I'm sure some people find a unit with two 'mechs, two tanks and two fighters very flexible.