I would think of the multi-site capability of the PSR500/600 separately from how you want to best set up your area programming. On these scanners the multi-site feature mainly concerns how the scanner "looks" at the control channels you have programmed into a particular TSYS object thusly:
Off: As most trunking scanners have always worked - Pick the first decent control channel and park there until it drops to unusable. (I have reason to believe the actual algorithm is doing a bit more than just this in the PSR500/600 but it's just suspicion for now; the official word is pretty much how I stated it and we'll stick to that).
Roam: The scanner looks at the control channels receivable by it and picks the one wherein the decode rate does not drop below what you set as Threshold Lo; what you set as Threshold Hi determines the "goodness" of the control signal that you have determined it should look for. For example, if you set Threshold Hi to 90% and Threshold Lo to 70% then the scanner will look for a control channel with a decode rate of at least 90% and park on that until it goes below 70% at which time it will attempt to find a better quality control channel.
Stat: The scanner will actually park on each decent quality control channel in succession according to how you have the "Check All CC" setting set. If "Check All CC" is set to "No" then the scanner will park on a different decent quality control channel each time it scans that TSYS. If you have "Check All CC" set to "Yes" then the scanner will park on each decent control channel in the list successively looking for programmed talk groups BEFORE leaving that TSYS and moving on to other TSYS's or conventional channels.
The idea, here, is that you can use Roam in a multi-site system so that, while moving, the scanner will attempt to lock on the best site it can find (according to how you set the thresholds) and stay on that site until the quality drops below what you have set in Threshold Lo at which time it will attempt to find a better site matching your Threshold Hi value, if at all possible. It's not a perfect system and has some limitations that, depending on your area and what other systems share those frequencies, can adversely affect the performance. However, if you move around a lot it may be worth experimenting with it (but see my note below).
Stat is intended to allow you to scan through all decent quality control channels while stationary (say, at home or work) so that you can hear all of the available traffic on all of the available receivable decent quality control channels. This can result in a long scan time, however. It will either take many successive passes through the TSYS to get through all of the control channels (if "Check All CC" is set to "No") and you may miss a lot of traffic on a busy system or it will take a long time scanning through the control channels during each pass through that TSYS (if "Check All CC" is set to "Yes") in which case you may miss activity on your other TSYS's and conventional channels. But it's all a compromise and it's up to you how to use these functions.
NOTE: As I wrote above, the system isn't perfect and does have some limitations. One notable problem with all modes (Off, Stat, or Roam) is that the scanner cannot isolate a system by its System ID, it only uses the decode quality as a filter, therefor, if you have other systems sharing some control frequencies with the ones you want to use within receivable range then the scanner might "falsely" lock on those systems and you may not hear what you wanted to; where I live, it is a major problem as I have many systems outside my desired area with good quality control channel signals that share some frequencies with the system I want to monitor. It most adversely affects Roam and Stat modes, since these involve the scanner most frequently checking other listed control channel frequencies but it can affect the scanner even when set to Multi-Site is set to "Off" due to the rapid changes in signal quality even while stationary (I think it is the many mountains toward my east causing rapid signal fades and phase changes on the strongest site's control channel output).
All that having been said, I think your question seems more geared toward how best to organize your programming rather than how to set up the PSR500/600's Multi-Site mode (though, of course, knowing how that mode works is still very useful!).
I looked at the UCAN system - it is indeed quite large! When reading your post I thought of how I set up my programming for my area. The system I monitor is also very large and has many sites covering a large area. It's here: http://www.radioreference.com/apps/db/?sid=499
if your interested. It covers two large counties in southern California and has many different users.
What I ended up doing was dividing up the area I wanted to cover roughly geographically using the labels for the talk groups as a gauge on where to include them. I set up two V-folders which roughly divide the area up into NORTH and SOUTH. Within each V-folder I have subdivided further into COMMON, COASTAL, INLAND, and EAST. These are actually individual TSYS objects. I can select these and deselect these using Funct+XX wherein XX is the number of the TSYS in the scanner's memory - I use the lower numbers for the TSYS objects to make it easier. I can temporarily or permanently lock out the TSYS's as desired according to where I am or what I am interested in.
Now the COMMON TSYS is what may be of interest and use to you in your situation. I originally did not use it and was frustrated that I had to copy so many talk groups between each TSYS - those talk groups not expressly used in distinctive "East", "Inland", or "Coastal" areas. This really began to balloon the memory usage until the obvious "DUHH!" moment hit me - add one more TSYS object which contains the talk groups common to all areas and not repeat them in each of the others! Adding a TSYS object only adds 10 blocks of memory usage in addition to the talk groups within it and is much better than trying to recopy three hundred fifty talk groups into each other TSYS! So the COMMON TSYS is always activated since it is applicable across areas while I can elect to activate the more area specific TSYS's as needed.
I'm thinking that maybe the "Common" TSYS idea might be helpful in your case?! I would find those talk groups you know are used throughout the area (state, in your case) and put those in the "common" TSYS while dividing up the other talk groups according to the areas they are intended to be most used in and assign those to specific regional TSYS objects. If you can't fit all this in one active memory program then divide the state in two in a way which suits you - say, for example, east and west or north and south - whatever works best for you. Just pick a usable dividing line that won't make it too frequently that you need to call up the other V-folder. Everything else is then fairly easily activated and deactivated by using the temp or permanent TSYS lockouts within active memory.
It would be nice if the PSR500/600 allowed talk groups to be assigned to multiple TSYS objects rather than just one but, oh well!
And you may ask how I use the scan lists. I use the scan lists as ways to subdivide by agency and user. In my case I use the following:
1) County Fire
2) County Law
3) County Emergency
4) County Medical
5) County Roads
6) County Misc.
7) State Fire
8) State Law
9) State Emergency
10) State Roads
11) State Misc.
12) Federal Fire
13) Federal Law
14) Federal Emergency
15) Federal Forestry/Parks
16) Federal Military
18) Rail/Bus Transport
20) General Misc.
I'm not sure if any of this helps but maybe it can give you some ideas!?