[Mobhack]

Formation exp and morale mods only apply at the purchase point.

If you cross attach say a militia element to a commando platoon, nothing special and magical happens to the militia element - you just have a formation with a new, under-experienced element in it. The new element may benefit from the better quality leader in the formation, is all. Same happens the other way - adding a commando unit to a militia formation gives that formation a unit with some better stats, but it will be lead (rallied etc) by the existing second-rate militia leaders of the formation.

In a campaign, experience is a thing that relates to the individual element's progress and how badly it gets beaten up along the way ec. and if it gets wiped out and has to be rebuilt, it gets new leaders from the default pool at standard exp and morale for the time frame. And the standard sort of leader for that element - a corporal or sgt if an individual element, a 2nd lt or 1st lt if it occupies a command spot.

So - especially with elite formations - keep the leaders of platoons, and especially companies, alive. Same applies for any line units that have been around a while - if a company HQ is destroyed and gets replaced by newly-minted 2nd lt Fluffy, it is less able than before.

But the SP games really don't have much in the way of "command and control" so the main thing is your long-standing experienced units and preserving these across the campaign.