Check the Combat Manuals: Lite in the Fan Articles forum. They will address your needs in most cases, and are semi-official.
I also remember discussing this with Herb, re WoB/C* Level IIs. 'Mechs and vehicles can mix easily. I'd exempt the "must have pairs of vehicles" for Level IIs. BA/infantry get grandfathered in with an idea I called "carriers". The carrier is what carries the infantry/BA into battle. You sub the carrier's movement stats, when needed, when building formations.
EG a if you want to have Purifiers in a Recon level II, put them in a Heavy Hover APC. Suddenly, the Purifiers move 10"+.
If the carrier has 1/0*/0 or less damage, then it doesn't count as a "unit" towards unit counts. So Purifiers in an APC are "1" unit. Purifiers in a Bolla are "2" units.
I considered allowing other properties to be carried over - eg. SR or MR damage, or armour. But that makes things a little too easy. You may also want to consider allowing the BA's "Type" to overrule the requirements applying to its stats - so a "Juggernaut" BA can be part of an Assault Level II, even if it doesn't have minimum 5 armour.
For the Marians, vehicles are the only issue - I'd change "must be pairs" to "must be at least pairs". So 2 and 3 is legit, 5 the same is legit, 2 and 2 and 1 isn't.
And round to nearest. 75% of 6 is 5, 75% of 5 is 4.
All seemed reasonable enough. But house rule whatever you're comfortable with.