I'm also using Squeezeslave on separate sound card and it mostly works ok. I just hear occasional hiccups (it's connected with Core being busy, but don't know how to deal with this).
I've also noticed that some files crash Squeezeslave (in my case I ended sending speech announcements to squeezeslave, cause as mono wav files they cause squeezeslave to die). I've asked on squeezeslave forums on how to setup on-the-fly transcoding of mono to stereo (have found command line that does that), but just couldn't get it to work...
If anyone knows more about transcoding on slimserver, we could solve that problem out also for other files...