I use an Arkel commuter with the older style mount (last years) on an older (circa '94?) Blackburn MTN rack. It works great. The Arkel mounting system is really adjustable, and allows you to move the hooks etc. The Arkel stuff is spendy, but well worth the money. I think you'll be OK.
As far as the un balanced feeling of a single pannier thing goes, I never notice and I ride with a laptop in mine. I notice it only when I'm walking it, or leaning the bike against something. Once you're rolling it magically disappears.