I think what you are trying to ask is, if your division has no artillery in it, but you have an artillery division, will the artillery division fill the support slots in frontage.
From my understanding, once the size of frontage is determined, the best unit (a division is a unit, and generally you will have nearly only division in a battle) is picked to fill frontage. All of its elements which will fit into the frontage (both combat frontage and support frontage) will be put into frontage. Elements which do not fit are simply left outside of frontage for this round of battle. If there is still room in either frontage lines, the next best unit is picked and put into frontage as above. This happens until both lines of frontage are filled, or you run out of units.
So if you have several infantry divisions, each without any artillery, and an artillery division filled with artillery, once the combat frontage is filled by your infantry divisions, if there is room for support units--since your infantry divisions have no artillery, all of the support frontage will be empty at this point--your artillery division will be picked to fill the rest of support frontage. If at this point there is still room in support frontage, other support units will be put into support frontage (engineers, pioneers, supply wagons, etc.).
The real issue is, as I see it, you want to get rid of those mamby-pamby 6lb-ers from your brigades and thus your divisions, and replace them with those manly 12lb-ers.
One could simply drop all artillery from brigades by changing the TOE of the brigades. Then you would simply have to buy artillery separately. The same number of infantry and artillery elements would cost the same, because the cost of a unit depends on the cost of all of its elements.
The drawback is in leading a these elements. Inside a division, it makes no difference, because whether an element belongs to a brigade or is independent makes no difference, bc the division leader is always the direct leader.
However, if you want to have a leader independently command lets say 2x Inf, 1x Cav, 1x Art , if these are all inside a brigade, you can combine the brigade with a leader. If you put a second leader on with higher rank (seniority) than the brigade leader into this stack, all of the elements inside the brigade will gain the leadership advantage of being directly commanded by the brigade leader (3% to power or something like that) with a stack leader above the direct leader.
But, if your infantry brigade is simply Inf-Inf, if you combine it with a direct leader and then add independent Cav and an Art units and then add a higher ranking stack leader to the stack, the 2x Inf elements in the brigade combined with the direct leader will gain the 3% advantage, but the Cav and Art will not.
My best solution would require 2 game-engine changes, albeit fairly small ones, plus a couple of RGD's:
1. Command Point cost should be assessed by elements and not units, if outside of a division. Currently units carry the CP cost parameter. EG a brigade with Inf-Inf-Inf-Cav-Art TOE will have a CP requirement of 5 to be fully lead, even if 1 or more elements are missing from the brigade.
2. Allow brigades to separate their Cav and Art elements out of them, similar to how you can break-up a division, only actually simpler. Combining them has always worked, as long as an empty slots is available. Then build all brigades to have an empty Cav and Art slot. This is actually also nothing new. If you look at the Union Inf-Inf-Cav units you can build in MO. you will notice that they actually have slots for 3x Inf(!!), not 2, so you can combine a 3rd Inf with these units. This is a very little known fact of the game that has always been there.
Now the player can build brigades, and if he wishes, build separate artillery and cavalry units to be put into the brigades, if he for example wishes to put the brigade under independent command.
If he wants to upgrade the artillery of these brigades, bc he originally put cheaper 6lb-ers into them, he can build some independent 12lb-ers, and when these are ready, he can drop the 6lb-ers out and replace them with the new 12lb-ers.
Or if the player wishes, at any time he could simply remove all the artillery form his brigades to consolidate them into an artillery division and fill the empty slots in his division with infantry. And if he wishes, he could take Cav units out of his divisions to send them on a scouting or other mission outside of his region.
The ONLY drawback with this solution is that if his 6lb-ers had gained a lot of XP through combat, the player would be replacing his highly experienced batteries with green batteries. But even that issue could be solved with some clever RGD's.
One solution to maintaining battery crew XP would be to have an RGD to upgrade 6lb-ers to 12lb-ers.
It would require the player to have a single stack of non-moving 6lb-ers in PP in a region to be played. When executed it would cost the player per 6lb-er battery in the PP stack half what a 12lb-er battery would cost, bc he would be paying for the guns only and not for raising and training gun crews. That the stack must be non-moving represents the fact that the gun crews would have to receive and learn their new guns.
The 6lb-er batteries would be upgraded to 12lb-ers (this is how it used to work), thus retaining a good portion of their XP. It would even be fairly realistic, bc the player would have to pay for the upgrades and plan for when to do it. He could NOT do it willy-nilly while his stack is maneuvering on campaign, which is how it could have occured when an event was upgrading 6lb-ers to 12lb-ers, plus it would cost the player to upgrade them and not be a huge free-bee as it used to be. For every 2 batteries upgraded like this, the player would get a light artillery replacement chit in his replacement pool.
To represent the fact that the player has all these 6lb-er guns left over from upgrading, a second RGD could be used to put a 6lb-er battery in training on the map from a light artillery replacement chit at half the cost (for raising and training the crew), to represent the 6lb-er artillery guns he has left over from upgrading being given to batteries in training. It wouldn't get spammed by the player to build cheap 6lb-ers from his light artillery replacement pool, bc buying a light artillery replacement itself costs the same as buying a 6lb-er battery IIRC, so the player would really only use this second RGD if his pool were filled with light artillery replacements from upgrading, otherwise he'd be paying double.
Sorry for the long post. Here's a potato smiley (^_^)