Might it be a good idea to do all air to air traffic on AM, thereby freeing up a few more FM freqs for air to ground work? Does a/g traffic ever happen on 168.20 168.60 etc or on the forest nets? They dont seem to have a lot of FM a/g freqs in California. And they dont use PL IIRC.
When I started with the U.S. Forest Service in 1974 all air to ground traffic was on 168.625 which used to be called "air net." It was also used for air dispatch, flight following, air to air and even dispatcher-to-dispatcher between communication centers. The traffic of some the latter did not always involve aircraft. Example when I worked on the Kaibab National Forest in the 70's I heard traffic such as "Coconino, Kaibab, what azimuth does Turkey Butte (lookout) have on our new start near Sycamore Canyon." Often only fire management officers or an assistant FMO would have air net radios (yes a separate radio). The use of 168.625 is now restricted to air emergency and brief initial contact if contact on other frequencies cannot be made and it is called "Air Guard." In my working experience, which includes four states, four national forests, three Forest Service regions and firefighting in 5 additional states on a total of 108 fires air to ground has not been carried direct on forest, BLM district and NPS park nets. Tac nets have never been used for air to ground, again in my experience, with some exceptions. Air to ground on forest net direct would tie up the entire forest including dispatch for new initial attacks as well as the other functions that use forest net, i.e. law enforcement, recreation etc.
Air to ground communications are usually between the IC and one aircraft (be that one helo, one air tanker or the air attack plane) or a division supervisor or crew boss with one aircraft. The amount of traffic and specifics can tie up the frequency for quite awhile which would preclude the frequency's use by all the other ground pounders. This would immediately present a safety problem.
As for the use of FM versus AM. The use of AM frequencies is governed by the FAA, not NIFC. AM is reserved for aviation coordination related traffic. The number of air to air frequencies is limited, more so than available FM air to air. FM air to air traffic is for fire tactics and can be monitored (ground units cannot transmit) by ground pounders on handhelds and mobiles. It is difficult to explain the difference between aviation coordination and air tactics relative to a wildland fire. Listening to the air attack organization on a large fire is the most effective way to learn the difference. You really can't use a scanner feed to listen to this difference as you need to see what frequency is being used and be able to control the scanner's scan and manual functions.
The Forest Service in California used to use 170.0000 for all air to ground. The BLM used 167.950 for all their air to ground. The BLM used to use 168.550 as "air net." This wasn't a problem unless fires were close enough to each other to cause interference with other high elevation aircraft enough that each aircraft could not hear the ground units because of traffic from other aircraft. If a fire got large enough that the NIFC cache frequencies were used another unique air to ground frequency would be assigned. Before the advent of programmable radios only one air to ground frequency was available as radios with lots of channels and the crystals to place in them were too expensive for the agency to afford them.
Beginning July 1st this year 7 air to ground frequencies will be employed by the Forest Service in California and they have been assigned based on four geographical areas. Only one frequency will be duplicated between these areas with enough distance and geographical separation to avoid interference. The BLM has been using air to ground frequencies assigned on four geographical areas in California for 2-3 years now. Large National Parks with exclusive jurisdictions (no state or local agencies with jurisdiction) often have their own air to grounds as they may tie up those frequencies for quite some time for search and rescue, medical and other non-emergency functions whereas the use of other nationally assigned air to ground frequencies is primarily for fire.
Elsewhere in the country air to ground frequencies are assigned on a dispatch center area basis. It appears that a standard list with a standard naming convention is now in use. So for example a frequency such as 166.7625 is called AG23 and is called such no matter what Geographic Area Coordination Center it is used in. It may be assigned as the primary air to ground for the Puget Sound dispatch center in Washington and the Grand Junction dispatch center in Colorado, but will be called AG23 in both locations. There are about 60 of these, but the complete list of them is presently not available to me. I will be doing some digging for these later.
In California the area each dispatch center covers is small in comparison with the rest of the country so they are not assigned for each center. I guess they could be assigned the same way as the rest of the country in theory, but there are probably not enough available frequencies to do it this way. California has a large number of interagency centers and a large number of Cal Fire only centers due to the amount of land with a heavy fire occurrence not adjacent to National Forest, National Park and BLM land. By the way, currently Cal Fire has only two air to grounds, but this will probably change in the next few years as more narrowband frequencies are available. One of their current air to grounds has only been available for a couple of years and is a narrowband frequency so it has only been available for narrowband capable radios.
I hope this answers your questions.