Current mods I am using: 2 Map packs, Lore, Armored Assault, Dynamic UFO Spawn
Playing the standard version (as opposed to CE)
Primary group of soldiers: 2 months, consistent gains in every mission of +1 to 3 to 5 stats +2 TU
Then I started deploying new guys. Yesterday 2 of my Corporals got promoted to Sergeants, their increase for that mission was +3 in every stat and + 4 TU.
Several times in past missions I saw low ranked soldiers gain +2 stats - something that has NEVER happened to any of my 'primary' team.
Right now it seems like low ranks get some crazy boosts in growth.
not sure if this is relevant - I normally deploy 2-3 rookies out of 8 with my veterans (Commanders to Captains).
As far as their activity during missions - the rookies do much less, they have only half the TUs of my veterans and spend much more of their time getting into position to be useful.
All soldiers are always loaded at least to 100% of their encumbrance - sometimes with a bit of penalty for particularly weak rookies.