>>2088254It's complicated.
Having some infantry joining an attack by armored vehicles helps protect from the random ambush chance where that rifle militia managed to destroy your heavy tanks.
Adding a single infantry battalion attacking from a different hex will give you more concentric attack bonus, which is one of the easiest to scale and most powerful modifiers in battle. It affects all units, not just the infantry.
Infantry can help draw away fire from enemy units with anti-tank capabilities, i think the way targetting is selected is random, so adding infantry meat can protect your vehicles, which are far less expendable.
You're right about keep formations seperate. All the part, infantry part tank formations are kind of ass. You're better off having them be seperate.
Anyway, regarding Heavy Grenadiers: They're only 800 men per battalion, which is important in a game where population is the most important number. They have 200 mgs and rpgs each.
rpgs are the most efficient unit in the whole game once you get to micro nuke rpgs, and mgs are absurdly tough for their price tag.