So I've had a couple other people try this, and it seems to be confirmed, but I figured I'd toss it up here as well.

As far as I can tell, adding new city guards and setting them up to enable/disable based on the civil war city posession doesn't work right. I put together a little 1-room mod to demonstrate.

I'd love to give you a download link to the demonstration mod, but this wacky forum won't allow me to post links . You can find it at shenkoheavyindustries (dot) com (slash) GuardTest.esp

The mod adds a test cell with 2 sets of guard spawners. There is a door to the test cell on the execution platform just inside the Solitude gate. One room has Imperial Solitude guards, one room has Stormcloak Solitude guards. If you enter the area on a character that has captured Solitude for the Stormcloaks it seems to cause one of the wonderful "Game closes with no error message" crashes.

The guards are set up the exact same way as the stock game guards. They're set to civil war soldiers, with EnableParents set to the markers in Solitude that enable or disable the stock game guards.

If anyone has any idea why this is happening I'd love to hear it. Or if you test out the little demonstrator and everything works fine. The odd thing was last night it actually fixed itself for me, then today went right back to causing crashes.