I've been considering a Self-Propelled Gun formation w/ Medium and Light Vehicles.
The basic element of the formation would consist of:
- 4x Medium SPG (Crew-Served Anti-Personnel / Medium Bombardment)
- 3x Light Vehicle (Logistics)
- 2x Light Vehicle (AA)
The formation would consist of three elements for a total of:
- 12x Medium SPG
- 9x Light Logisitcs
- 6x Light AA
I think that would be a defensible artillery formation.
First of all, flavour- and RP-wise, do whatever makes you happy and/or fits into your story.
Mechanically, you shouldn't create such units for three reasons:
1) Vehicle based supply can work with any formation in its chain of command and putting it into Front echelon is just wasting it - the supply trucks might get shot up before being consumed.
2) MB can work from Support echelon but CAP only works from Front echelon. Regardless of which stance you use with them, 50% of their capability is wasted.
3) The Light vehicle with AA is going to get hit fairly often so you might as well make it a Medium vehicle to give it more armour. At Front echelon, it can pull double-duty, but at Support echelon it can only defend this formation, nothing else, making it likely to remain passive.
Now, it's perfectly valid to make SPG formation that is armoured and still stays in Support echelon, because the armour protects its from enemy counter-battery fire, orbital bombardment and ground support fighters. Similarly, you can make a perfectly valid Front echelon SPG formation.
Here are my proposals:
Front Echelon Bombardment Formation:12x Medium vehicle (max armour) with Light Bombardment + Crew-Served Anti-Personnel modules
6x Medium vehicle (max armour) with Light Anti-Aircraft + Crew-Served Anti-Personnel modules
This formation can use all of its weapons at every combat round with the LAA pulling double duty in attacking ground support fighters in addition to firing ground-to-ground mode. It will use double supplies though! With medium armour, the vehicles have some survivability without becoming too expensive.
Support Echelon Bombardment Formation:12x Medium vehicle (max armour) with Medium Bombardment
6x Heavy vehicle (max armour) with Heavy Anti-Aircraft
This formation can remain in the Support Echelon and still bombard most enemy formations as well as support Front echelon formations. It's AAA element can hit any enemy fighters regardless of their mission or target, so it'll fire each round as well. With armour and their location, they have a good chance of surviving combat. As a cost- and size-cutting measure, both vehicles only have 1 module on them.