The 2500 would work for the Integra/Ontrac 2 system.

The 2500 would not work with the Insight and the Trimble Autopilot system.

Both Ag Leader displays are very forgiving and can work with a variety of GPS sources.  It is the Trimble Autopilot system that prevents the 2500 from being a useful GPS source for your situation. So the guy that told you "no way" is correct.

Since the Autopilot is a genuine Trimble product it needs TSIP (Trimble Standard Interface Protocol) as the communciation standard from the GPS receiver to the Nav II.  This means that you would need a Trimble GPS receiver of sufficient quality as the GPS source for the Nav II.

If the Insight is unlocked for Autopilot then the Insight can communicate back and forth to the Nav II. 

Here's my personal situation:  I have two tractors each using Trimble's Auopilot system with the Nav II.  The Nav II is the brains of the steering and is a silver box without any type of display that is usually mounted beside the seat. 

In my 8120, I have an older Trimble 332 GPS unlocked for Omnistar.  The 332 is connected to the Nav II.  The autopilot unlocked Insight is attached to the wiring harness for the Nav II.  It communicates back and forth with the Nav II.  The Insight is the display for the steering (among other things) and communicates my desires for steering to the Nav II.  The Insight receives GPS information from the Nav II and not directly from the GPS.

This works very well.  The Insight does a great job of organizing my AB lines and other field information which is not necessarily related to the steering.  The Insight can handle steering  my tractor, running the clutches on the planter, monitoring my starter fertilizer and controlling my preeemerge spray. I could, but don't have it take over the duties of the population monitor (I still use my old Deere 250) and hydraulic seed drive if I had that.

On my 8310, I have a Trimble Auotpilot as well.  In this tractor, I have a CFX-750 which acts as both the display and GPS source.  The CFX-750 communicates with the Nav II in that tractor.  Since these products are all Trimble they work together.

I sometimes have need for GPS in another Insight that may be used in that tractor.  I can get NEMA GPS (which the Insight can handle) out of a port on the CFX-750.  When I'm doing this for NH3 application, the CFX-750 is doing the steering and providing GPS to the Insight which is handling the NH3 application.

I am not familiar with the AFS 372 but my guess is that it is a Trimble GPS branded for C-IH.  It may have GLONASS capability.

My guess is that it can be set to output TSIP and/or NEMA messages.  The TSIP messages would be needed for the Trimble Autopilot with the Insight.  TSIP would be all right for the Integra but NEMA would cover both the Integra and the Ontrac 2.  Mayabe TSIP is acceptable for the Ontrac 2. 

I suspect it will have two ports.  One could be setup for NEMA out and the other for TSIP out so that swapping between the two vehicles would be fairly easy.

This may be one of the few choices that would cover your situation of wanting to have a GPS with GLONASS that works with these Ag Leader displays and the steering options indicated.